Stick a finger in the tubes and feel for "rifling". That'll identify SLX/SPX and TSX. Which also indentifies Columbus. There were manufacturers who were religious about tubing choices - all Reynolds, all Columbus, etc. It's one avenue of elimination in addition to:
1. BB shell width and threads, cutouts and stamps.
2. Lugs
3. Serial numbers.
Don't rely too much on dropouts - as that's one area where even the most religious of manufacturers were willing to cross a "border".
Even lugs are borderline in that regard.
=8-)
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5000+ wheels built since 1984...
Disclaimer:
1. I do not claim to be an expert in bicycle mechanics despite my experience.
2. I like anyone will comment in other areas.
3. I do not own the preexisting concepts of DISH and ERD.
4. I will provide information as I always have to others that I believe will help them protect themselves from unscrupulous mechanics.
5. My all time favorite book is:
Kahane, Howard. Logic and Contemporary Rhetoric: The Use of Reason in Everyday Life