Is this a kit? A scratch build? Your own design?
Best thing to do with an established design is to start with the default thrust angles. Don't worry, the plane won't be unflyable if the thrust angles are off by a couple of degrees. There's a trim guide on the NSRCA website,
http://www.nsrca.org/trimA.htm, which will tell you how to trim your plane out for aerobatic flight, including thrust angles.
If you're working from scratch, you really need a means to measure the angles. It's not difficult if you know a little trigonometry. A flat stick or plywood disc installed in place of the propeller makes a good basis to take measurements. The measurements are a little difficult to describe in words; maybe someone has a link to a diagram? Worst case, start with the motor at 0 degrees. Again, the plane won't be unflyable.