Can a bad steering rack and pinyon be diagnosed before having to replace the power steering pump and pulley?

That depends on what the problem is. Fluid leakage is very evident, and the steering gear is the most common suspect. A loss of power steering assist is a little trickier. The pump can wear out and fail to develop the really high fluid pressure needed, or there can be internal leakage inside the steering gear that lets the pressurized fluid bypass the "power piston". GM owners from the late 1980s are very familiar with that disaster. There is a tool that can be installed into the high-pressure hose to figure out which part is causing the problem. It involves closing a valve for a couple of seconds to see if the pump can build pressure. If it can not, it's worn out. If it does, the steering gear is suspect.

The problem is that special tool is extremely uncommon because it rarely gets used. In ten years at a very nice new-car dealership, I was the only person who knew what it was for, and I only used it once. Without that tool, it is more common to replace the easiest and least expensive part first as a test. That's usually the pump, although that can be the harder one on many import vehicles. Often experienced mechanics will go for a part based on what they've found in the past on similar models. In that case the steering gear is the more logical choice.

Replacing random parts is the least effective and most expensive way to solve a problem, but there are times when replacing the wrong part first is less expensive than a long drawn-out diagnostic procedure.

One common clue to internal leakage inside the steering gear is temporary loss of power assist. A worn pump will not suddenly work at times. A worn steering gear can and often does as with those GM cars. Typically you'll have no power assist when turning one direction, first thing in the morning, for perhaps as much as half a minute, then, once the fluid warms up, the power assist comes back for the rest of the day. Each day it takes longer and longer for that assist to return, then it begins to affect turning in the other direction too.

You might find more information in this article:

https://www.2carpros.com/articles/power-steering-problems

What kind of problem are you having with the steering?