Jon wrote in post #7445618
Or you could ask the pro either in advance or when the day arrives.
I would go with this option if I were in your shoes. I've been to a friends wedding in which I didn't know the photographer and didn't have any contact with him during the wedding and felt very very awkward shooting. Every little look I got from him was wierd, and the whole experience was just uncomfortable. (His pictures sucked btw and in retrospect I wish i would have just talked to him and then been more comfortable getting up and shooting so my friends could have had some decent pictures).
On the other hand, I was also at a wedding this year where the pro photogs were a friend and his wife. So i had very open communication with them and they pretty much invited me to take all the photos I wanted, and helped me with ideas and concepts It was a much cooler experience but they were also comfortable telling me to get out of their way because we had established that they were the pros. I also held flashes and reflectors for them on the group shots. They appreciated the extra set of hands and I got to learn a little about what wedding photography is like.