I have found 3-4 gr difference in "same brand, same lot" .38 sp brass and as Coyoteking says with different brand brass have found double digit brass weight differences. You can reliably find a "double bullet seated" case but I sure wouldn't rely on weight comparison to find a suspected double charge in a small pistol powder charge weight.
I see where you're coming from but even with CF rifle cases a spread of 3-4 grains is a lot.
It's a good place to start and not nearly as random as picking a handful of cartridges out of a batch.
UPDATE
fingers284, you really piqued my curiosity with cartridge weight. Soooooo just to scratch that itch, I thought I should check out what the differences in case weight would be between 10 different types of brass, out of my 9x19 bin.
I picked out 5 of each brand, three of which were obvious milspec.
The milspec cases were all 5grains, +-.3 heavier than the the commercial brands. The milsurp cases were IVI, WCC and PMC, which is usually very close to milspec.
Their average weight, over 15 cases was 60.02 grains. 59.8 was the lightest and 60.3 the heaviest. Closer than I expected.
The commercial brands, WW, FC, RP, Norma, WIN, Imperial, Dcc, were all an average of 5 grains lighter, but with a larger spread in weight.
The heaviest were the old Winchester Western at 56.8 and the lightest were the Remington Peters at 54.5.
I rounded everything off to the nearest tenth of a grain.
I will definitely concede that if the OP can't tell the difference between milspec and commercial cases (sometimes difficult) that to weigh the loaded cartridge to find the overloads, if any, wouldn't be the best way to go about this.
OP, get a kinetic puller and pull all of your last batch of reloads and any other suspect batches. It will take a couple of hours but well worth the time and components.