what is the best way to diagnose harley wobble? i have a 71 shovel and the front end starts to shimmy above 55 mph.

I would start with tire, rim, is it true, bearings correct spacers and tight. Ask again If none of those. They are the easy ones and you have to start somewhere. Another question would be when did you start noticing the problem, or did you but the bike like this.
DEERSLAYER

i bought it that way.

Losen the pinch bolts on the triple trees see if it unsprings the front end. Somtimes over time the front end dives and doesn't always come back all the way a little at a time and that will cause a high speed front end wobble. Losening the pinch bolt will unsprung a front end if this has happen.

Seeing you bought it like that you should inspect the entire swingarm and front end, because you haven't knowledge if the bike was rebuilt correctly. Before that I would replace the tires front and rear, you can always use new tires and when they are off the rims have the rims checked for trueness. Check the bearings and the axels, and the torque on the axel nuts when taken off the axel nuts. Check everything until you positively find the problem, problems. Pull of the front end and swingarm and check the against the Harley manual? I know it sounds like a lot of work but it is something you should fix if you ride above 55. What model is the FL?
DEERSLAYER

When my air suspension gets low I have a problem with speed wobble in the curves. Long sweeping turns if there is a little bump in the road it will send the bike into an oscillation. I keep the suspension pumped up to 10 psi - when it drops below 7 it starts to get the wobbles.
I also noticed that since I put the new cam in as I decelerate - riding no hands - I'll get a front wobble. I concluded that the bike now vibrates more at lower revs and the bars will oscillate with the engine making the front end start to wobble.
The reasons I say this is to relate the different causes of my wobbles and hopefully you can use any of that info to help diagnose your problem in case it isn't a tire, wheel or rim.