Aware,
There's no special trick to it, oher than hiring a costly private detective. Many totally inocent people look very guilty, and many deeply guilty people are skilled at looking wholly innocent.
I understand how you can interpret the clues you describe in such a way as to feel suspicious, but I agree totally with volcano --- I guess those behaviours would match my performance too, and I'm not having an affair with anyone.
Above all, here's just no substitute to talking together about this. Gently, gradually, tactfully, raising your concerns, after asking her about some of these things you've noticed.