It's perhaps easier to tell the lower level models for some makers. If you see a tubing sticker on one of the main tubes saying "Hi-Ten", "High Tensile", "10xx", "20xx", or "Carbolite" (on Peugeots), it's one of the lower end models. Seeing the same on the fork (but having something else on the main tube) is an indication that it isn't one of the top-of-the-line bikes from the maker (but not necessarily a reason to pass on the bike).