"You don't know"= you don't know some information about something Example: 1.You don't know how much this dress costs, do you? "You can't tell"= you are not allowed to reveal information 2. You can't tell anyone my secret. "you can't tell the difference"= you don't know the difference between 2 or more things. 3. You can't tell the difference between these 2 handbags can you? One if fake and the other is Gucci.

"i can't tell" MAY mean the same, as "i don't know", but it MUST NOT mean the same