A checklist of some key features of my Speedmaster CK 2998-6
Clockwise from top left:
Correct dial with applied Omega logo with correct font. The dial has only "Swiss Made" at the bottom. . This should be an obvious one, but you often see these with later dials.
Double step caseback with nicely preserved Seamonster; again, often a later back is fitted (single step) with the Hippocampus practically polished away.
The bezel, one of the commonest problems; the delicate and fragile bezel with the 70 and its dot in the right format (dot at '4 o'clock' not underneath the 70 as on the later bezels.)
The cal 321 movement serial number of 18xxxxxx puts it around 1961 to 1962 correct for the later 2998s.
And the stamping inside the caseback should look like this:
Hands are the correct 'Alpha' style with the simple baton style for the sub-dials, and the cute spear type with the correct rounded opposite end shape (not the later straight end style).
Bracelets weren't available to me but you should look here or here for guidance.
Finally note the 4mm pushers; so hard to find, many have been replaced with the later and larger type.
Surfing through the resources listed in the previous post as well as the above links will save you from missing these kinds of details. Good luck in your search for your dream Speedy.