Most Helpful Guy
Of course you can tell. Yoga pants aren't much thicker than a coat of paint, so anything you're wearing underneath would be visible, even IF the yoga pants weren't partially transparent, as many are.
Now, I don't find VPL a huge sin like some people do, but let's be real: the ONLY reason girls wear yoga pants is to show off their ass. You can say "oh, they're comfortable" all you want, but lots of clothes are comfortable without being shrink-wrapped to your ass. I'm totally fine with it if you want to show off your ass, but let's not try to pretend they're for any other reason, UNLESS you only wear them to work out.
I'm normally a fan of regular bikini-cut panties over thongs, but even I might say "thong" when wearing yoga pants. Because, if any clothes in the world were designed for thongs, it's yoga pants.2