Yeah, it's the sunlight thing that makes it hard to know without someone else telling you. In the gym, mirrors let me see when my shorts are getting see-through in the same light that anybody else is going to be looking at them. Outside - no mirrors. Maybe you could "moon" your reflection in a store window.

A riding buddy of mine showed up one ride with brand new kit that was totally see-through. I didn't have the heart or the chutzpah ... hopefully one of our male riding buddies gave him the word.