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.

There are other indicators, that may help, possibly the most significant of which is the seat post size which will tell you the wall thickness of the seat tube.

Otherwise, the best indicator is to know the maker, and what the specs call for.

So, for example, show me what appears to be a high end British or US made steel frame, which takes a 27.2mm seatpost, and I'll very willingly bet a few beers that it's Reynolds 531 or better tubing. Likewise you might see a mark on the steerer if you remove the fork.

BTW - many (most) makers of top end steel tube sets, mark the tubes themselves. One place to find such a mark is at the bottom of the seat tube, a few inches above the bottom bracket. Besides identifying the tubing, the marks also tell builders which end to cut (or not) when cutting tube to size.