Context

I am designing a town for my play group to eventually travel to after they learn 2 NPC allies of theirs disappear after promising to come back from the town in question and they discover the NPC's wagon off on the side of the road, clearly attacked with all goods and the NPC's taken. The trail goes cold and knowing they aren't far from a town, they go to the town only to discover that it's basically a refugee encampment of people hiding from a Lamia and her Jackalwere. I have devils basically running the town holding people hostage for their own amusement with a clever human merchant having struck a deal of tribute with the Lamia in order for people to survive so long as they sacrifice so many people in a month.

Actual question

So here's my problem, Jackalweres (MM, p. 93) have the Shapechanger ability that allows them to shapeshift into 3 forms: their true jackal form, a specific human's form, or a hybrid form. This shapeshifting is what allowed them to infiltrate and destroy the other local villages in my campaign, however I need some way for my PCs to defend Honava (the last village in the area) from infiltration in some practical and believable way.

How can my players combat the creature's shapeshifting?

Restrictions

My party will only have access to Lvl 3 spell slots. I have a Bard of the college of whispers, a Fighter who is a Monster Hunter, a Warlock who made a pact with a Great Old One, and a warlock who made a pact to become a Hexblade Warlock.

None of my players have access to truesight. Due to how I have the area set up for the level my players will be at (Level 5 or 6), no devil (CR 5 and lower) in the area has truesight, and true seeing is a 6th-level spell that my players aren't at a high enough level to have access to.

My main question is this:
What other methods are there for dealing with shapeshifting/polymorph effects at lower levels?

Use silver and truth

You say that the devils have made some sort of safe zone, but they are secretly dealing with the Lamia on the side. However, they, being devils and well versed in the idea of binding clauses, don't truly trust her, and why should they? Lamias are chaotic evil, they can't be expected to uphold their end of the contract.

So instead of just letting everybody in willy-nilly, you set up your little refuge with a wall, and only a few gates that are permanently manned. If you want to get in, you have to agree to holding a bunch of silverware in your hand while solemnly swearing you're a Jackalwere, before you're let in. Obviously this is a lie and humans might find this lie awkward, but to a Jackalwere, this statement would be physically painful. Alternatively, have people quickfire-answer questions that are objectively true, something a Jackalwere might not even be capable of doing, because it would have to consciously not lie.

While per pure RAW monster statblocks this won't truly harm a Jackalwere, the fluff around were-creatures has always been that being in contact with silver is painful for werecreatures. (It might be for devils as well, depending on your setting fluff, so make sure the border patrol are humans.) While Jackalweres are not true werecreatures in the traditional sense, they do seem to suffer from the same weakness to silver:

Immunities bludgeoning, piercing, and slashing damage from nonmagical attacks that aren't silvered

In addition, the Monster Manual states that Jackalweres were specifically made to lie, and that speaking the truth is physically painful for them. Thus, a perceptive guard should be able to seperate humans from Jackalweres by making them interact with two sources that cause them physical pain.

Once your players have been to the place once they can pick up on this idea and start using it as well, paying strangers they meet only in silver or handing them silver-coated objects to see their response, while asking them questions that they know the answer to, to force the Jackalwere to either lie or be in pain.

That way, even though they don't have any truesight, they'll still be able to determine with reasonable certainty that a creature is a jackalwere, while still allowing you the flexibility later of having a surprise Jackalwere with an extreme tolerance for pain or the like.

Use the Truth Like a Shibboleth

tl;dr The players ask straight forward questions and watch for a pain response.

Jackalweres Can be Discovered When Speaking the Truth.

The description of the jackalwere notes a non-magical way of discerning the creature's nature.

From the 5e Monster Manual (p. 93):

A jackalwere is born to lie, and perceptive creatures might notice it wincing in pain when it speaks the truth.

Narrative

The players have come across the information, "those whom it pains to speak the truth are not what they seem." This could be introduced on a hastily written yet ominous sign.

Examples

The players ask simple unambiguous questions, such as what is the color of the banner they're holding, how many fingers they're holding up, or on in which hand they're holding their sword.

If they're specifically looking for a painful reaction, give it to them immediately. If not, make it a low perception check (DC 10 or 12) to notice the jackalwere wince when answering.

Use non-silver weapons

As mentioned in another answer, Jackalweres have the following immunities:

Damage Immunities Bludgeoning, Piercing, and Slashing from Nonmagical Attacks that aren't Silvered

It does not necessarily follow from this that Jackalweres have a lycanthropy-esque aversion to touching any silver. After all, many creatures have this immunity condition. Can ghosts not touch silver?

Asking a jackalwere to tell the truth is very flavorful, and unlike holding a pile of silver, this is specifically mentioned as a trait. Probably the most fun solution, but also time consuming and with a risk of error, because of the perception check requirement.

For a cheaper, easier, and RAW solution, the players can do the inverse of the silver test. Line folk up and give each of them a small prick on the arm with a steel blade. If they don't bleed, jackalwere beware!

There may be other options too, and it can be fun to let your players surprise you. Allow them to research jackalweres in a library, to learn their basic traits, and then see what creative problem solving they can do on their own. Making sure that there are at least a couple of solutions available to them is very good due diligence on your part, however, as it allows you to drop hints in case they get stuck. In addition to the above ideas, your players may seek out a special magical item, spell scrolls, or a creative combination of spells not mentioned here.

Don't come up with a solution, trust your players to be creative and reward them

I'll propose a solution as well, but trying to develop a 'solution' that your players will then need to discover and use is likely a recipe for failure.

Railroading your players to find your solution can be very problematic if they don't pick up on your 'clues'. I can't tell you the number of times I thought I was explicit in what needed to happen to increase odds of survival and how often they just didn't get it or did something else.

What you need to do is consider possible ways that they could solve this to make sure it's doable. Once you've done that, you need to trust in the creativity of your players. Let them develop a plan. If it's a good plan, let it work. If it's not, then help guide them as much as you can without railroading them into feeling like you solved their problem for them.

A possible solution

Creating a daily 'gate' that every citizen must pass through. As each of the creatures passes through, they can pass some of the tests created by other stackizens as answers to this question.