How do I interview or access a potential teammate? What are the things to look out for and what are the appropriate questions to ask?
FYI: I am a Frontend Developer

ask them to vertically center a div

I might be the last person to give advice but:
since you're not primarily looking for just a coder but a teammate you might wanna test that.
E.g. "Given that [common situation in team] how would you react/resolve that?"
"If you want to do [thing that should involve communication but could also be done lone-wolf-style] which steps you'd take?"
Im not talking "If your teammate pisses in your coffee..." but rather day to day interactions that should be a no-brainer for good teammates

stuff like that to see if he's able and willing to cooperate and understands why one needs that stuff (e.g. why document, why discuss your ideas with your team, ...)

The last interview I took was basicly that. I answered it based on my DevRant noledge