You can call Colt and give them the serial number or you can look it up in the Blue Book of Gun Values.
if it's a 380 caliber then made about 1924
Automatic Colt Pistol. Colt invented the .45 Auto and thus had the right to name it.
What is the value of 1903 colt pocket pistol.32 calibre?
Caliber .45 Automatic Colt Pistol.
@1914
100-800 USD
about 1920
@ 1972
The caliber of this weapon is .45 ACP, or it's full name is .45 Automatic Colt Pistol.
If it is a Colt Vest Pocket model, 1921.
@94 yrs old
somewhere around 1973