(someone) can tell (clause)
When you "can tell" something, it means that you are able to know it by seeing it, tasting it, feeling it, etc.:
I can tell that you're getting better at it.
Could you tell I wanted to leave?
You can leave "that" out in casual conversation, but in formal speaking or in writing you should definitely include it:
I can tell that you put a lot of work into it.