The length is everything when it comes to rarity and value! There are many Colt 16" uppers and barrels flying around but very few 14.5" barrels or complete uppers.
For authentication:
- Proper barrel profile (Gov't, with Grenade launcher cut)
- Bayonet Lug
- Proper barrel markings "C MP 5.56 NATO 1/7"
- "C" marked upper (behind spent casing deflector) OR stylized Diemaco "D"
- "M4" marking above the gas tube
- Tapered front sight pins
- Parkerized under FSB
- Extended feed ramps
- Chrome lined bore and chamber
- "C" marked bolt carrier
- "MPC" marked bolt
- Small take down pin holes
- Numbered top rail
- Auto sear relief cut forward of the rear take down pin hole
- Side sling swivel
This is by no means a complete list, but it's a good start. All of the above would be seen on a real Colt M4 or 6921. If you do have a complete upper, it is possible that it came from a 6921, but more likely you have a pieced together upper from Colt parts.