To comprehend, discover, or discern (e.g. I can tell what is wrong.); recognizing the truth (e.g. I can usually tell when someone's lying to me.).
When it comes to the meaning "can identify or recognize", the signer fingerspells the word "tell", not signing it. If signing "tell", it means different, which means "I can inform or communicate".
Gloss: CANNOT TELL: you never know; one can't be certain. E.g. "I couldn't tell what they were thinking."