Many Blu-Ray players like my Panasonic, have an auto mode that will detect which is the best resolution format for your TV. In most cases with this mode, it will send either a 1080p or 1080i signal even to a 720p TV. However, as Scott suggested, you can set it to 720p to see which format looks best. Personally, I have found that the 720p setting is a bit soft, therefore selecting auto or 1080i and letting the set down scale it may provide the best results.
Ian