The easiest way to determine whether a Colt rifle was manufactured prior to the September 1994 federal assault weapons ban is to check its serial numbers. According to Geremy Chubbuck, writing for AR15, Colt rifles are stamped with serial numbers that indicate whether the weapon was assembled prior to the date the ban went into effect. These stampings should always be verified with the manufacturer.
Apart from serial numbers, pre-ban assault rifles have certain cosmetic features in common. The 1994 ban identifies assault rifles largely by their rate of fire and outward resemblance to military-style weapons, according to Wikipedia. Assault rifles automatically extract the spent cartridge and replace it with a new cartridge. Semiautomatic rifles that meet the criteria for the federal ban must have detachable magazines and at least two of the following features: a pistol grip, flash suppressor or threaded barrel, bayonet mount, grenade launcher mount, or a folding or telescopic stock. A rifle assembled after September 13, 1994, that meets these criteria is considered an assault weapon for the purposes of the law, and likely falls under the statute. As always, the surest way to determine whether or not a specific rifle was assembled prior to the ban taking effect is to contact Colt and request information.