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.
Use "by___" to talk about the evidence that you see:
You won, didn't you? I can tell by your smile.
"By ___" can come before or after the clause:
I can tell by the color that they're almost ripe.
I can tell that they're almost ripe by the color.