There may be more, but you can always count on there being one under the seat to determine that you are sitting there, and one that is activated by the clutch pedal if you have a foot clutch. Those are the ones that cause the most trouble. Either one will keep the tractor from cranking.
Other places to look are the PTO actuating lever or knob, and the parking brake. The fuel system won't have a specific safety switch if it has a manual on/off lever on the fuel filter, but some models also had a fuel solenoid that would act the same way as a safety switch...that is, if the solenoid didn't actuate for some reason then the tractor would crank but not start.
rScotty