When nothing is happening, pressure coming off the pump impeller that isn't doing anything pushes the relief valve open, allowing it to recirculate back the reservoir, so you'll see it stirring around in there....
When you turn the wheels, fluid level should drop a bit as the high pressure flow is diverted to do some work. Fluid will still circulate, as it's pushing fluid return back through the low pressure hose now, but you'd probably notice a change in direction, on most units.
Depending on the reservoir design, you might have to suction out enough fluid so that you are seeing right into the reservoir cavity, rather than the neck...but I'm thinking you have the old square box-type reservoir with no neck, so you should be already looking straight into it, even when full.
If no circulation at all, something really catastrophic has happened, like I said the pulley could be spun on the shaft, or the pump impeller may have done something similar inside the pump. I'd probably try cracking the high pressure line open a bit and verify that there's no pressure from the pump, if it's accessible with the pump mounted up. No pressure, a new pump is called for.
If the impeller went bad, I'd definitely recommend flushing the system, the lines and rack may be full of shavings from the disintegration. They'll ruin a new pump, and probably the rack, in short order.
If you have circulation and pressure, the problem is likely the spool or a line blockage. If the spool went bad, it's new rack time, unless a blockage is the problem...and maybe even then.
I don't really know any good way to check for a blockage other than to remove the lines at both ends and see if they are clear by blowing through them or whatever. Or crack the high pressure line fitting slightly at the rack, see if you have strong pressure there....if so, I'd say the rack is shot.
Return line blockage is probably not likely, but you might check to make sure there's not a screen in the pump side of the return, I have seen it before, I don't know if these have it though. You take the return line off and you can see it in the hole and can usually remove it for cleaning. I've seen this more on imports though...
GM racks usually go bad starting with morning stiffness, but not always. If it has a blocked orifice inside you might be able to blow it clear with compressed air through the line fittings (from low side to high, I'd imagine) but I never tried, I'm just guessing.
I do remember seeing a couple of pumps on the older GMs like yours just fail suddenly, I'm hoping that's your problem...a lot easier than a rack replacement.