How do you know if tofu is extra firm?

Excuse me, could you please elaborate on how one can accurately determine if tofu is of the extra firm variety? I've noticed that there can be quite a bit of variation in tofu's texture, and I'm curious about the specific characteristics or methods one should look for or use to confidently identify extra firm tofu. Is there a particular label or packaging indicator that I should be aware of? Or perhaps there's a tactile test I can perform to make this distinction? I'd appreciate any insights you might have on this matter.