Weren't you given an indication when you asked your prof/advisor to write you a letter? If they write positive things, usually they respond with a grin on their faces and say, "I'd be HAPPY to!" And actually, usually when I ask for rec's, my professors act so flattered that students like/respect them enough to ask for it from them.
When I went to my premed advisor, she told me she only had good things to write about me. Usually your person gives you hints and reassurance before they even write the letter. If they don't think they can write you a good rec, they'll warn you.