I'm working with a new senior programmer who has almost the same amount of experience as me. He his own project to work on -- but I have to make sure he does not mess things up. 

Now, how can I monitor him since he's not junior? 

Do I examine his code? Cons of this are that I am not as deep into project as himself so it's time consuming for me (have my own project besides this). 

Do I only do QA from the perspective of another senior? 

Any advise here is appreciated. I was recently in the same situation where I relied that another senior will handle all tickets properly, and then it took me 3 weeks to fix his oversights. So I do not want to be in the same situation again.