How do I handle it when I think someone is probably, but not positively, lying to me?

I have never known how to deal with lies. When someone lies to me deliberately, as opposed to a mistake or misunderstanding, and I catch on, I really don’t know how to handle it.

I’m not talking about great big terrible guilty lies, but minor things that are almost trivial in themselves. Almost, but not quite, because over time they harm the relationship.

For example, I have a friend who often wants to reschedule dates we’ve made. That’s not a big deal. There are plenty of times when I don’t feel like keeping a social obligation either, although I usually do anyway. If she just said, “I’m just not feeling up to going out today,” I’d understand.

But when she starts recycling excuses—like the second or third time when it’s “I have to take my sister to the doctor, and that’s the only time they could get her in” or “I have to have two teeth pulled tomorrow, so lunch is out” or “I have a migraine and it might not be gone by then”—I start thinking: (a) If she doesn’t want to keep our date, that’s fine, we can call it off; but why keep rescheduling and then breaking it? and (b) When the same excuse keeps cropping up, why must I pretend to be stupid enough to believe this story again and again?

That’s what really bugs me. Frequent changes of plans can be irritating, but that’s not the issue here. It’s that I see only two choices. Either I have to play dumb and act like I believe her or I have to confront her with what seems like a lie by the third or fourth time. How would I even do that? —Especially when I don’t know for sure, and it isn’t worth trying to prove anything.

I’ve had similar situations before with other people, situations where I can’t just ignore a harmless excuse because some response is required of me (change of plans, expression of sympathy or concern, finding a substitute, etc.). I object to being forced into a corner where I have to either call someone out or make believe they’re fooling me.

How do I say something back that lets them know I don’t buy it, without turning it into an unpleasant confrontation? I would like to have a smooth and preferably tactful solution to these situations that doesn’t leave me feeling insulted or used.