Spirit level = typical bubble level
Right, but as you said earlier SEEING it would likely be a problem, so a mercury switch can indicate level remotely - via wires and lights.
The more I think about this the worse the problem(s) appear (-:
There could be as many as three planes to consider;
Horizontal - i.e. what a spirit level would indicate.
Tractor's wheelbase plane,
ground plane that the pallet rests on.
and from my (admitted very limited) experience of this I would choose a good side view of the pallet and forks.
Traditionally this has been a helper, or I pull on the steering wheel and climb forward off the seat to try and SEE where the forks are.
If I did a LOT of this I would set up a wireless back-up camera on a tripod.
For unloading trucks or trailers it would be quick and simple to set up and aim at the rear of the vehicle, for pallets that are scattered around - tough, but I would either have to climb down and position it to see the pallet, or find a way to mount it to the fork frame.