How do you tell if your fly hook needs sharpening and how do you sharpen it?

On a recent outing I found that I was missing strikes, I would try and set the hook but they always shook off... not a complete miss but a strike and 1 second later off. So I started thinking that maybe my fly was dull; it was a used fly, don't recall how many times I had used it. So I switched to the exact same fly but brand new and bang I was hooking up. So was my hook dull? I can't tell any difference.

Doug