A few times I've seen bulges that could easily have been guns, but the give-away was the hawaiian shirt and the constantly scanning eyes.
Honestly, I consider the whole package when I'm taking that stuff in.....what's he or she wearing? How is he or she acting? That stuff is as informative to me as the bulge is. After all, if the person did a good job of concealment, even an aware person might not pick up on it. So when I see certain clothing styles and the way the shirt is worn...stuff like that....
then I start looking for a telltale bulge. I seldom notice the bulge first. And as always, I'll concede to myself that it could be nothing more than a cellphone or other benign object. Unless it is REALLY obvious, you just don't have any way of knowing for sure.
One of the other guys who is a member of my Wednesday morning business referrals group always carries a commander sized 1911 in a Fobus paddle holster under a sport coat. He doesn't seem to care if anyone notices the prominent angular bulge in his jacket when he leans forward at the table. I tease him about it regularly.
As far as I know, he, my wife, and I are the only ones who carry there.