The two engines have different oil pans, among other changes.

Before the oil pan method was identified, the only way to 'tell' if you had a 7-bolt engine was the production date of the car. According to the staff of Conicelli Mitsubishi, cars produced between March, 1989 and the third week of April, 1992 have 6-bolt motors; May, 1992 to the first week of May, 1994 have 7-bolt motors.

Another way might be to check the front brakes. 7-bolt engines are associated with big brakes; cars that have big brakes often have 7-bolt engines. Unfortunately, nobody seems to know if this method is reliable or not.