Depends on your phone. The LG phones I have used with my Autocom do not ring through the headset. I keep my phone in my LT's "oddments box" so I can't see the display.
Sometimes I can hear the little "click" when the phone auto-answers and connects the incoming call. Sometimes. Usually the way I know I have a call is when the confused caller says "Hello?".
I figured out the better way to detect an incoming call purely by accident. I was riding along listening to my iPod, singing along with the music. I think it was Bowie's "China Girl", a song I like but probably can't sing well. That was the moment that my wife called. She evesdropped on me for a minute and then began laughing at me. I told her that was a breach of etiquette, like walking into the bathroom unannounced when someone else is singing in the shower.
OK, so now all I have to do is sing along whenever I'm riding, and just listen for the laughter.