How hard is too hard?

I'm a first time oral sex giver, and I'm unsure of a few things. When giving oral sex to a male there should be a certain amount of sucking. I'm worried about leaving painful hickeys on my boyfriend's penis, and I need some advice. How hard is too hard to suck? Also, how will you know when the guy is CLOSE to his release? I understand the concept of ejaculation, but that is DURING the orgasm. I would rather know when it was about to happen so I could prepare myself. Is there a way to tell?