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.
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?
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?