anthony,
where the main line from the hyd pump connects to the hyd control valve check that fitting, it should be a largish 90deg elbow where the main line/hose connects to the control valve. If there is an 1/8in pipe plug in it that is where you connect your gauge to to check hydraulic pressure from the pump. It may also have a oring plug or a cap in the main inlet valve section where you can connect a gauge, there will be the same for checking the secondary pressure as well in the opposite side of the control valve in the auxiliary sections. There are also a main and secondary relive valve with an adjuster nut on top where you can remove the top and check the spring inside it and see if it is broken if need be.
If you do not find any plugs to connect a gauge to then you may have to do it the old fashion way by installing a Tee fitting inline where you can connect a gauge that way. (yes you may have to visit a hydraulic shop to get those, if so make sure you know the size of the fitting or remove it and take it with you so they can get you the correct one). Yale was pretty good about providing a test port in the inlet section and the auxiliary section so you probably won't have to go through all that.

Low pump pressure can cause steering problems because the steering gets it's pressure from the control valve that goes through a priority valve located down on the transmission area. The pressure from the pump should be around 2200 secondary to 3100psi at primary relief pressure. I'm not specifically sure the exact pressure because i do not have the service data for that truck, but the trucks in that series generally had close to the same pressure ratings, they haven't changed that much over the years.
Maybe someone else here does have the service data on this DE series and can correct me if i'm off too much. (just remember the pressure should never exceed the hose ratings and those should be printed along the hose).
Oh yes, almost forgot to tell you, make sure your test gauge is at least a 5000psi rated gauge.