Well Hag covens typically form in 3s, so thats a good clue to hang onto when trying to narrow down suspects. Each type of hag usually fits a certain terrain and type of psycological trauma they are suited to, so that can be helpful in figuring out what type of hags youre dealing with.
You can also set bait, especially children or particularly beautiful/handsome individuals. Fey make for potentially useful lures too, and might actually be pretty willing to help you.
Anything with access to truesight or resistance/immunity to charms would be a good first step to combating a hags typical MO, magic resistance even better. You could get some good mileage out of a swarm of imps doing recon work with their flight and invisibility. A rakshasa could be an interesting and powerful ally if you could gain access to one.