There are various ways people use, but the one you want to stay away from is using the marks on the swingarm. I figure you know that or you probably wouldn't have asked, but just thought i'd mention that. I use 2 holes that are in the swingarm on each side. You will find one next to the front belt guard mount and on the other side in the same place. There are 2 on the left, but DON'T use the one with the metal collar in it. I already wrote about this twice and the rest it should be obvious. but i'll just say use a bolt or something in the hole with a string around it and measure to the center of the axl, then take it to the other side and do the same. i am working on using a straight rod, bolts or some sort of pegs in the holes, then lay the rod against them and that way you can tie a string around the rod out from the swingarm so there are no obstructions and i can get it more exact.
That said, i feel when very well aligned the belt rests too hard against the outer flange, so i've been aligning it then adjusting the left side of the axl in a bit to relieve that pressure. I really think triumph's final drive on this bike needs refining and i have no doubt that will happen on future models, if it hasn't already.