Well, luckily, it's not just any character that can be lying to you! There are statements that if you see, you know they are ground truths:
- If you see a Baker claim "I am the original baker", then they must be a good Baker. When a Baker is lying/corrupted, they will always claim that they were originally another Villager, and will never claim the above statement.*
- An Alchemist that claims "I cured 0 corruptions" must also be a good Alchemist. When an Alchemist is lying/corrupted, they do not actually have the ability to cure corruptions. If they were to claim "I cured 0 corruptions", that would be the truth, which evils/corrupted cannot say, so anyone who does say that must be good.*
* except if they are like a puppet or something but that should be relatively clear to see if you come across that
- The most notable of all is the Confessor, who cannot lie. Ever. If she says she is good, she is. If she says she is dizzy, she is either evil or corrupted.
About corruptions:
- Look in the deck view (top right) to check which characters are in play. You can then put a number on total sources of corruption. For example, if you see that Pooka is in the deck, you know that there can be at most 2 liars in the circle who may be actually good, and they must be next to an evil player. If you have 3 people lying in a row, it is likely that the one in the middle is Pooka corrupting her neighbours.
- Outcasts, currently, cannot be corrupted. No corruption sources can touch Outcasts yet. You can use that to narrow down where corrupted characters may lie. So if a Plague Doctor claims that a person is corrupted, and they're lying, they're evil.
- If you execute the Drunk, don't worry - for nearly all situations (unless you have a Lilis also in play), the Drunk is essentially a free execution.
Sometimes it does take a bit to unravel the mystery of a given puzzle. If you see conflicting information, why? Are they evil, or simply corrupted? If you think they are corrupted, there must be a source of that corruption. Can you come up with an explanation that doesn't contradict anything in the circle? If you can't, then they may just be evil.
This might be a bit waffly, but hopefully this helps a little.