I wouldn't say that counterweighting is always the answer. If any part of the linkage isn't stiff, that's where I always start. Arrow straight push rods, heavy duty linkages, no play in the clevise/horn connection. You should not have any play or flex when you try to move the control surface by hand.
Spotting which surface is fluttering is often impossible. I saw it in my old Goldberg Extra 300 shortly before the stab exploded and once again when the light was just right on the aileron of the Hots. Check all surfaces for slop and flex. Check how much you can twist the ailerons.
As for geometry and mass of the counterweight, I made up a set with just a guess. Good old That Looks About Right. Worked but as I mentioned, after a few flights I finally got another flutter. Replaced the weights with the next size up and that was that. I also guessed where to put it on the aileron. Since the root was very solid and the aileron stock was a bit flexible, I put it out towards the wing tip as shown.
That was the 6th Hots I'd built through the years but the first one with so many flutter problems. Solving it was great fun.
Dave