Some of it is trial and error (and a spot of luck), to be sure.
Some of it is this thread
on the Gearbox forums. (EDIT: Ninja'd with link to the same thread.
The unaccepted sidequests should all be available after completing Talon of God. Which is, conveniently, also the point at which unaccepted sidequests snap up to your level in P2.5, so ...