Asked to help interview a candidate for the first time and I have zero idea what I'm doing

Been at my job barely a year and my lead just asked me to sit in on a technical interview next week and 'give my read.' I still feel like the junior one here.

What am I even supposed to be evaluating — whether they get the right answer, or how they think? I'm terrified I'll either be too soft or accidentally tank someone who was just nervous, because I know exactly what that feels like. Anyone been thrown onto the other side of the table early? What did you actually look for?