Your friend is learning to play the guitar. He's frustrated because he doesn't think he's improving. But you do think he's improving, you say this to encourage him.
I can definitely tell you're getting better.
When you "can tell" something, it means that you know it by experiencing it:
I can tell that you're lying to me.
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 definitely tell that you're improving.
When you use "definitely", it means that you're sure.
"Definitely" is often used together with "can tell":
I can definitely tell that you've lost some weight.
"Get better" also means to recover after a sickness or injury, by the way.
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