Only way you will know if the person you invade is hollow is if they aren't wearing armor. They'll still show a full health bar regardless if they really are at 50% or not.
I've been invaded A LOT while hollow. I think you don't notice it as much because of the Life protection ring nowadays though.
Wait, so you guys are saying that someone can in fact get straight up invaded if they're hollow? I had absolutely no idea if that's the case.