Cut one open. If it slices easily with no sign of a center woody core they are fine. But even if they are a bit woody they will still cook and/or can ok. The processing tenderizes the cores quite a bit or you can always cut it out.

Dave

I found that if they are bigger than a softball, they are more likely to be tough. Dave's right, they will tenderize even if they are bigger.

Pull the biggest ones, you'll be able to tell by digging near the stems with your fingers (sort of like checking radishes). By pulling the bigger ones, you'll be able to let the little ones still grow.

Beet seeds will produce multiple beets, so thinning should be done (not that I always get it done).

Thanks!