How can you tell if a used (junk yard) power steering pump works?

You can't do that. Without power steering you won't be able to turn the steering wheel. In the old days, it used to be power 'assisted' steering, but now the steering pump does all the work. You would fail at turning the first corner.
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?Get a friend to drive you to the junk yard.
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Try spraying some lube or wd on the universereal joint under the dash
if that doesn't work change the two lines high and low pressure line

need a change of fluid-it gets old and thic n works only wen warm--may also need a new pump--junk yards are an option

I would drain your power steering pump, even if I had to take it off and dump the fluid! Yes, you do need to use power steering fluid, but what will get you in the most trouble is if you mix your fluids because then they work against each other.

While some people will use transmission fluid in a power steering pump in an emergency, I always recommend they drain it out and use the proper fluid in the proper place.

Call the local Chev dealer or local salvage yard. If that fails check Ebay

WHY BUY A JUNK YARD POWER STEERING PUMP 12 YEARS OLD SEALS PROBABLY CRACKED DRIED UP. 94 TOPAZ HAS SAME ENGINE 2.3 2300CC. FROM 1990 TO 1994 HAS SAME ENGINES.

All depends what is leaking. For power steering leaks there is not much to do but replace the leaking part. If it is a hose it is not a big deal. If it is the power steering pump itself check autozone, etc for availability. They'll probably have to order it in for you. Sometimes you luck out and the leak is due to a seal where a hose is screwed into the pump or the rack and pinion unit. I believe some auto parts places carry stop leak for power steering units but I have no experience with them. One last thought is if the car is pretty old and the leak a slow one, you just check your power steering fluid periodically because you can buy a whole lot of power steering fluid for the price you'll pay for a power steering pump. You can also consider a replacement from a junk yard. Generally power steering pumps out last the cars they are put in.

ask dealership if pulleys are intechangable with your model/year/engine. call junk yard, someone might have dumped a new pulley. try a slide hammer at the junk yard. you need a straight pulley if you want belt to stay on without premature wearing. add some P.B. Blaster to stuck parts to try and help you break them loose. .

You can try a junk yard or better yet the dealer for new.

Check with the junkyard where you got the pump...you may need to change the pump bracket with the pump, or use a different pump.
You don't have a separate tensioner the pump is on a slide mount and is the tensioner (why the parts guy is a parts guy not a tech).