Suggestions for testing a never installed, 20-year boxed amp.

You can turn on a solid state amp without a load connected to it.  But, just to be safe, you should contact Pass for their recommendation.  I would not hook up any speakers until the amp has been turned off and on several times and has been allowed to run for a while with no obvious signs of distress (the most obvious sign of trouble will be a strong odor of something burning, repeated blowing of fuses or tripping of any safety relay that cuts the amp off).  

The concern with an amp like this sitting a very long time unused is the state of the electrolytic capacitors in the power supply.  With tube gear, you can soft start them by attaching them to a variac and using that to slowly bring up the voltage which allows the capacitor to "reform."  But, some solid state gear do not like under-voltage conditions so this is not recommended unless Pass Labs says that this can be done.

Thanks. I appreciate your advice. 

Before I powered up my long dormant Pioneer SX-1050 I found a current limiting device on eBay. It used a light bulb to someone limit current. It powered up OK. I then used a multimeter to check for DC on the speaker terminals. Luckily there was none. 

Yes, good advice about checking for DC with a multimeter.  Unlike a tube amp with output transformers, some solid state amps can pass DC.

I’d contact Pass Labs. They may tell you to run the amplifier through a variac in order to allow the capacitors to reform safely. Best of luck. 👍