Stuff you CAN see as you walk up (from the outside):
VS omits the black rubber gaskets under the horn grills and rear deck lid grill. IM (and I think Beck and JPS) includes those.
Also, the VS Speedster seat cushion is covered with two stitched pieces of fabric, unlike the original which has no seam running across it. I know IM is one piece, JPS is two pieces, but I forget what Beck does.
And once you've seen IM (or original) square weave carpet, you'll recognize the difference immediately between that and what VS uses. (But original Speedsters had rubber floor covering up front.) For that matter, same goes for the overall upholstery quality, too. There are reasons IM's take six months to build. VS carpeting is only slightly more durable than butterfly wings.
Another interior IM 'tell' is that they go to the trouble of building a small box under the pedals so they look like they're going through slots in the floor like on the original cars. Nice.
And, of course, the most foolproof tell - if the owner is wearing a real Rolex, the car is not a VS.