I would weigh 4 pills on each side for the first try. If there was any differences I would eliminate the 4 ones on the lighter side. (If there was hopefully!)

Then I would put 2 on each sides and hopefully see a difference again, thus eliminating the 2 lightest ones once again.

I would then be left with a pretty good idea of which one was safe or else I would just fucking guess at that point. :P

I'm tired...