Here is one way , but it requires a blank wall behind the tail. Get a pc. of White Posterboard, draw a horizontal line on it with a Felt Tip Pen, from the center of the line draw a Line up at 1/2 the V-tail angle and draw another one in the opposite direction.( make these two lines extra long) With the wings assembled on the plane Put a Level big enough to span the fuse and allow you to get the wings level .( you'll need to put a pair of equal sized blocks under the ends of the level, and make sure the center of the level is over the centerline of the fuse ) Stick the posterboard to the wall , trying to get the center of the V inline with the centerline of the fuse. Go around to the Nose of the plane and see if the Horizontal Line you drew on the paper lines up with the level, if not adjust the paper and check again. Once the Horzontal Line on the paper lines up with the Level , you can now a least get the two tails at the proper angle.
Now getting the Incidence correct is another issue.

Jim B