How do you know you are giving good head?

The surest way is to ask. If you're giving a guy head, you should be able to talk to him about it.

It is a good thing for people to signal to their partners when something feels good. That way, their partners know what to do more of, and what not to waste time on. So encourage the guy to vocalize, moan, breathe fast and deep -- that sort of thing. Tell him it turns you on! (You are turned on, right?)