Weigh it. If frame and fork, stripped of everything except the pressed-on fork/headtube races, weigh 3 Kg or less (maybe a bit more than that for a frame that large), then it's almost certainly Reynolds 531 double-butted.

For instance, I have a 1980s Raleigh 57 cm frame that's Reynolds 531C DB: frame is 2015 g and fork is 832. A 1970s 61 cm Bob Jackson that's also R531DB is somewhat heavier at 2195/862. Those are forks that are on the heavy side (though I am positive that they're butted blades, because they both have stickers). A Motobecane from the 80s, has a fork that's under 700 g, while a Guerciotti fork from the same era is 768 g -- both of those are Columbus tubeset frames. At the other end of the scale is a 70s Falcon 63 cm frame with seamed, straight-gauge tubing has a frame weight 400-600 g more than these, and fork weighs an additional 100 g, for a total over 3.6 kg.

Seatpost size can tell you something too. Reynolds DB seat tubes are actually only butted at the bottom and generally take a 27.2 mm seatpost. But with a venerable frame with a "history" may have been outfitted with a too-small post along the way, and the top of the tube may be ovalized or otherwise misshapen. The size of the tube somewhat below the clamp, where the tube is likely not messed up, is a better indicator of what size post the frame should take. Aforementioned Falcon was a case like this, and ends up taking a 26.4 mm post. A good bike shop will be able to diagnose this -- using old posts of varying sizes along with hammer encouragement to get tightish ones out when driven in. Note: a large frame like that might have a straight gauge seat tube, even if the rest of the frame was double-butted.