Motorcycle Questions? Ask a Mechanic.
The first check would be the balance of the wheel assemblies and the radial and lateral runout. If you have spoke wheels you should also check for loose spokes. If this is something you would like to try yourself I can help you. The runout measurements can be done on the bike. The balance takes a stand of some sort (there are commercially available stands on the market) with roller bearings on it.
Once you eliminate the tires and wheels as the problem the next check would be the rubber motor mounts. They can deteriorate over time and they lose the ability to dampen engine vibrations.
I hope this helps. Please let me know if I can be of further assistance. Thanks, P.J.