Review to understand, not necessarily to find problems. If you do find bugs and anti-patterns along the way, great, ask about them. But consider it your goal to learn what they did, why they did it this way.

When you're giving feedback, phrase things as questions most of the time. Idk about you, but I'm rarely so sure of myself. If you're saying stuff like "Would it work better if we tried xyz?" instead of "Change this to do xyz.", people respond better. Sometimes it starts a discussion, sometimes you learn they had a good reason for what they did in the first place.

It'll be really tempting to skim and just give it a quick Looks Good To Me. Try not to.