An easy way to tell if it's original equipment or not is the location of the shifter. A factory 4 speed shifter operates the gears directly, and will have a long round tapered steel handle coming out of the floor dead center near the front of the hump.
On a 3 speed with a floor shift conversion kit, the shifter handle will come through the floor well to the left of center and close to the front of the seat. This is the result of the shifter being mounted externally on the side of the transmission behind the main gear case. Linkage and arms then control the transmission via the pivots that originally connected to the column shift linkage.
As noted earlier, a floor shift conversion isn't necessarily a bad thing. Factory column shifters were frequently unreliable as they got older, especially with youthful drivers that tried to speed shift them.
