squirt a decent amount of lubrication on the steering column UJs in the engine compartment first though.
ideally jack the front wheels off, get a mate in the car and really work the lube into the UJs lock to lock. you should see lots of rusty mess coming out of them.
once you are sure these are nicely lubed and freed off, then test the steering wheels off the ground. how does it feel? engine on any different?
then back onto the floor and try again.
as sierrafery says make sure youve got fluid for starters. when doing your 'on floor' test look in the reservoir whilst mate turns lock to lock, should see fluid rushing in and out of reservoir. some air bubbles on surface but no foaming.
if foaming, try bleed the steering box. in fact **** it, bleed the box anyway. you'll be amazed how many systems i find air in even though its kinda self bleeding.
once youve got that sorted, yeah youre looking really for a relation between fluid pressure and ease of steering, if pressure drops when steering is heavy then youve got a pump problem, otherwise suspect box, although this is unlikely.
main culprit for irregular stiff steering is column UJs.