I think you can call the airline and ask if any pets are on board the flight. They should be able to tell you. Your biggest problem will be if the plane is full and you need to switch seats -- be prepared that you'll probably be the ones that end up moving (fair or not).

Also, you should make sure you bring medication with you however, just in case. The people with pets are not supposed to remove them from their carriers. If they follow that regulation, you're probably fine and will not even realize they are on-board. If someone pulls their pet out of the carrier to pet them or whatever, then the fur flies literally.