By it's serial number.Go to proofhouse.com to find the date of manufacture.
Your colt 1911a1 was made by colt in 1945.
1944
In 1944.
made about 1917
It was produced in 1942.
made by Colt 1913. not unusual if you have a springfield slide on your Colt 1911
About 1969 with that serial number for a Colt goldcup national match .45
made by Colt in 1918. not unusual to see a springfield slide on a Colt frame
made by Colt in 1918. not unusual to see springfield slide on Colt frame
made by Colt in 1914. not unusual to see a springfield slide on a Colt frame
it was made by Colt in 1918. If there is a springfield slide on the gun the it was put there later on. Not unusual to see a springfield slide on a colt receiver
made in 1916 by Colt. Your Colt has a Springfield slide, which is not unusual.