...just from looking a it?

If you are wanting to ID a Colt factory upper assembly you can look first for the barrel markings with earlier ones being, "C MP Bore", "C MP C" or something similar to indicate that it is a Colt barrel.(Very early ones are a bit tougher to ID and you should go to Don's web site for that info). Later barrels can be indentified by the "standard" NATO markings which would be something like "C MP 5.56 NATO 1/7" or similar.
On the upper reciever you can, unless its a  relatively early one, look for the forge markings that are a part of the reciever itself. On SP-1's the forge marks (if present) are usually located below the rear sight adjustment in the thin "web" or connecting area of the carry handle. Typicly a "C" or "CM" was the usual marking.

On the SP-2's it could also be located in the same area unless you had an A2 upper in which case the forge markings were located to the right of the rear sight adjustment knob. The markings range anywhere from "CM", "C", "CB", "CMH", "CMF" and more than likely one or two more I do not recall at this time.

And if you are lucky enough to have found an upper that came off a weapon, then you could look for the Proofing stamps that would be located above and to the right of the ejection port. There you would see at least one stamp indicating that the upper had survived one shot from a high pressure proofing round.

Hope this helps a bit.

Edited for spelling

Don's web site could i get that plz thanks in advance

Try this: AR-15 Model History