I obviously don't have a level that's 15 feet long to check that the beams are exactly straight from one side to the other and don't veer off either way. The end walls are not perfect either, so I can't just take a measurement from there. Plus, as I stand there looking up at the ceiling I won't be able to tell if one is crooked until it's actually mounted. Too late. Even if I laid on the floor I don't know if I'd detect if the beams aren't exactly square to the walls, ceiling, and the other beams.
How do I do this right the first time and get everything square?