This is a common expression, both on its own and as a part of a larger sentence.

Be careful with it, because neither “you” nor “tell” means what you might think at first.

“You” here is the generic you; it doesn’t mean the speaker is actually talking to someone. Spanish often uses either “tú” or the reflexive in these cases.
“Tell” does not mean “say” here, but rather “Notice”.

So, “You can tell” means that something is noticeable. Examples:

You can tell she’s been working out—look how big her biceps are!
Can you believe he’s incredibly rich? You could never tell by looking at his shabby clothes!

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