Speaking for myself from my own experience, it's the motion that gives away deep cloak to me. Sometimes the inf will turn or make some subtle movement that gives him away. Sometimes it's my own motion while looking in just the right spot. If I strafe or move at the right angle in relation to the deep cloaked Inf, I pick up the distortion. Not always. It's easy too miss. But that's usually the case when I do catch one.