Guys mastered the art of the hand job back before they ever had their first girlfriend.
So this guy I have known for awhile and even had a thing with was sitting in the hot tube too.
So a good way to tell? And they certainly don't compare to intercourse or to oral.
But unless you've got serious skills, chances are that kind of dual action won't happen for too long -- it's a little like rubbing your belly while tapping your head.