How to get force applied to object during FixedUpdate

How are you checking for triggers?

The triggers are simple box colliders attached to the surfaces.

The problem that I think you have is that whenever you collide with a trigger, you add a force, so when you collide with two or more, you add the force multiple times.

What you could do ins$$anonymous$$d is have a boolean, shouldLevitate, which you set to true whenever you collide with a trigger (and set to false when you aren't). If the boolean is true in your FixedUpdate, then apply the levitating force, otherwise don't. Doing it this way will only apply the force once, no matter how many triggers you collide with.

As far as I know, there is no easy way to get the net force on an object.

Yes, _Gkxd that is exactly the problem.

Unfortunately, the forces are not applied by the object to itself, but by other objects acting upon it (think of conveyor belts under the object that are lifting and moving it).

The object applying the force does not know what forces other objects have already applied to the object it is lifting.

Ins$$anonymous$$d of having those other objects actually apply forces, have them queue up their forces and then have the main object deal with them in FixedUpdate().