On the topic of weights, frame without fork should weigh 1600-1700 grams for 753. A 531 frame w/o fork should weigh 2000-2200 grams. Depends on size of course. Weighing is the most reliable way to distinguish the two tubesets. Measuring diameter is less definitive.
Even this is not completely reliable. A 753 tube set will weight the same as a 531 "Professional" (aka "Special Lightweight") tubeset, and variability in the weight of lugs, fork crowns, and bottom bracket shells will further obscure things.