im going soon to a local non profit bike community spot where I may be able to pick up an old bike or frame.

How do I know which frame is 26 vs 27.5 vs 29? (Most bikes there may not have tires or the right tires to clarify)

Also, between a 26 or a 29, which can take 27.5 tires? Or better to ideally stick to a 27.5 frame?