If you are non-reving there are no guarantees! I've been in a seat at the gate and pulled off when a last second pax showed up. You will never know until you are about to push from the gate.

That being said, there are ways to look at the bookings and you should be able to get a good feel for whether or not they'll make it.

The key is to be flexible. You may have to drive to BWI, fly to atl, then catch the ATL flight to FRA. I just took two buds to Munich but we had to drive to an airport 80 miles away, then fly into Stuttgart, and then take a train to Munich. That's part of the fun!