On a butted tubeset, the main triangle has double-butted tubes on the top and down tubes, but the seat tube is typically single-butted: the top of the seat tube is not butted, but as thin as the center portion of the tube. Example: A Reynolds 531 seat tube that's 28.6 mm in diameter takes a seat post 27.2 mm in diameter. The difference is a wall thickness of 0.7 mm, which is the thinner of the wall thicknesses (the thicker being 0.9 mm?).

Often, the quick-and-fairly-reliable way to tell if a frame is made of butted tubes is to measure the seat post hole, or the seat post; if the post is, in fact, not undersized. If measuring the hole, I take several measurements at different locations, because it's fairly common to find the top of the seat post has been ovalized somewhat, or a lot, if too small a post was fitted/clamped.

The other way to tell is to weigh the frame. I have several frames the same size, and one is plain gauge steel. It weighs from 500 to 600 g more than the ones constructed with butted tubes. That's not so much really, in terms of total bike weight, but it's about a fifth of the bike frame weight, so hard to miss.

Reynolds made all varieties of tubing: plain gauge, single-butted, and double-butted.