You never know (what you can do) till you try.
Prov. Even if you think you are not able to do something, you should try to do it. Jill: Want to go rock-climbing with me this weekend? Jane: Oh, I can't rock-climb. Jill: How do you know? Have you ever tried it? Jane: No, not really. Jill: You don't know what you can do till you try. Alan: I'll never be able to learn to dance. Jane: You don't know till you try.