Hmm - I have to think about this. If you say -"he's been checking up on me", this means he has been keeping a regular watch on me. It implies suspicion. "I'll have to check on that" really means the same as "check" by itself. In fact, the extra "on" doesn't sound good. I can't think of any use of the phrasal verb "check up". I think you can safely ignore it. If we go to the doctor for a routine appointment, we call it "a check-up" - this is a noun. Hope that helps, Michael