How confident can I be that the 25 year-old compressor in my truck has the proper amount in it right now?
Unless you know the vehicle's service history you can't be 100% confident; however...
If the system is presently properly charged and lubricated, once shut off the oil tends to remain in the major components in a very consistent manner. As long as the system has remained leak-free, the oil level in a particular component isn't going to become "incorrect." These components include the compressor, evaporator, condenser, and receiver-drier/accumulator. It's not like the oil remains in suspension inside the lines or hides elsewhere. My minivan FSM describes it better than I can:
When an A/C system is assembled at the factory, all components except the A/C compressor are refrigerant oil free. After the refrigerant system has been charged and operated, the refrigerant oil in the A/C compressor is dispersed throughout the refrigerant system. The A/C receiver/drier, A/C evaporator, A/C condenser and the A/C compressor will each retain a significant amount of the needed refrigerant oil.
Refrigerant oil must be added when an A/C condenser, A/C receiver/drier or A/C evaporator is replaced (see the Refrigerant Oil Capacities chart).
That's why every single FSM I've ever seen has virtually the same procedure for oiling a new compressor -- drain and measure the old, refill the new. In addition, most FSMs will have a chart like this to help when replacing specific components: (NOTE - this chart is NOT for your vehicle...it's only an example).
You should be able to search online for a similar chart for your truck. As Chris stated above, if the inside of the system is contaminated all bets are off and you should flush it all, followed by a complete system re-oil with the proper amount of PAG46.
If the system is clean/uncontaminated, I'd measure and fill the new compressor accordingly and add 1 oz. to the drier/accumulator and 1 oz. to the new condenser.
A little bit of extra oil certainly won't hurt, but too much will tend to adversely affect cooling.