You do the same thing as when checking out any tool/library/whatever: Skim the README, then if still interested jump into a file or two and look at what the code is like. It’s pretty likely - if this person works in the same domain, usually the case - you (as an experienced dev) will be able to guess what files are worth looking at and quickly get at least some minimal understanding of what is being done.

And remember, this is just to decide whether to invite someone for an interview. Looking at the repo is just to get some extra signal.

I’m surprised this needed to be explained.