Originally Posted by
FBinNY
Actually, there's no reliable way to be sure. Weight may work to separate a good quality frame from one of much lower quality, but things get close near the upper end, and variables like lugs, BB shell or whatever make it hard to distinguish between butted and high quality plain gauge tubing. Also, know that both butted and plain gauge tubes come in various wall thicknesses, further muddying that water...
An industrial xray machine would let you know for sure. There are many companies that offer xray inspection services. They could tell you if your frame was made from double butted tubes. Unfortunately that would be an expensive way to satisfy your curiosity.