by trial and error. the fuji film nearly can't be overdeveloped. if you add a minute to the time estimated for development, it hardly changes the result. the fuji just finishes development when all the exposed colour is transfered. there are even people who claim that they take the sandwiched pictures home and separate them then. I did not try this yet. if longer development doesn't change the result, they are underexposed.
The gist of what xya has said is that you can easily rule out underdeveloping by massively increasing the development time without affecting the end result. Then with trial and error experience you can judge between overexposure and underdevelopment for yourself.
I'm with the others, if you're developing in the correct time frame, even adding a minute considering it's self terminating isnt going to change the look of your image. If you develop a little over what you're meant to and it still comes out looking over exposed, it's more than likely over exposed.
If you overexpose and separate the film quickly this will give you overexposure and underdevelopment but this will also often give uneven development. It's instant film so experiment!