One way to know for sure is to measure the diameter of the pivot pin. Then you'll know for SURE! 0.312 inch pin diamter is a large hole lower and 0.250 is a small hole lower. Colt manufactured large and small pivot pin lowers. The there are large and small fire control pin lowers to boot! Confused?
In theory, ANY upper should work on any lower as there are adapters to convert large hole uppers to fit small hole lowers and vise versa. That being said, I meant it as a general statement of suitability. In reality, ALL uppers will not work on all lowers because of mechanical differences, tolerances, design requirements, etc.