The approach is programmed into the FMC navigation as a sequence of waypoints. It is aware where either the approach or transition to an approach (manually selectable from the CDU) begins.
Let's say if you only select the approach in the FMC, the typical waypoints would be CF36-FF36-RW36. In this case, CF36 is the first waypoint of the approach.
If you however select transition to an approach (not arrival), it will look typically like ABCDE-FGHIJ-CF36-FF36-RW36. In this case ABCDE is the first waypoint of the transition/approach, as far as the FMC is concerned.
The easiest way to see whether the FMC is "on approach" mode is by checking RNP - it changes to 0.3 when FMC transitions into "on approach" logic. This assumes arrivals/approaches with standard RNPs.