Give them a let out clause

If you feel the lie is about something important, then look for external validation of what they are saying. Without hiring a private eye, check, if you can, that they were with Fred last night and not with the sultry blonde from the office.

For some liars, it may be almost a relief to come clean. If you’re pretty sure they are fibbing, then confront them. But rather than: “You are such a liar!”, take responsibility for your own doubts: “I’m having real trouble believing this!” Then tell them exactly why you don’t believe them.

They may try to reassure you or become angry because you doubt them (and both reactions could be a sign of lying or not lying). Tell them that you’d rather hear the truth because none of this makes sense to you. Resist the temptation to hurl accusations, which can be used by them to change the subject onto how you never believe them. Even if they don’t admit the lie at this point, it’s enough that they know you are doubtful.

So to spot a lie, you need to:

Listen to how much they say - if they’re not saying much about something, they may be lying to you.
Listen for how much they distance themselves from what they’re saying by not connecting to it with “I, me, me, and mine…” Also, do they describe how they feel about what they’re discussing? - Not using feeling words may be further evidence of self-distancing.
Notice whether they’ve suddenly developed a super-memory for lots of details about which you’d normally expect someone to be naturally vague.
Notice if they seem over-eager and relieved to change the subject.
Pay attention to whether they’re overly defensive.
Remember that not being told important stuff is still a lie.
Confront them when you are pretty sure you are being lied to. Give them the opportunity to come clean. And remember it helps to have external evidence that what they’ve told you is an untruth.
Finally, don’t feel bad about having been taken in by someone; as we’ve seen, telling if someone is lying isn’t as easy as we all assume it is. When you trust someone, you give them the opportunity to behave like a reasonable human being; if they then waste that opportunity that is a reflection of them, not you.