Before I spar someone who is new to the sport, I let them know that they set the intensity. I usually just say, "hey, we'll go as hard as you want to go."
Its tough when you're just starting out, you want to gain some respect, you want to show off all the cool stuff you and your bro's worked on in the garage... etc etc etc.
I just let them know if I have something I want to work on, or ask if they have something they want to work on.
if not, I just let them know that they set the intensity. If they're going at me 50% I'll give them 50%.