The 'official' oil PSI test for the engine was/is to loosen the bolt MCRider mentioned above (or a head nut, don't have the manual to see which one), then run the engine to see if oil gets there within 45 seconds (cold). But, that presumes the top end is already oiled...
You can open the valve covers and put some oil right on the cams and rockers, front and back. You can also run the engine with those covers off, to make a special mess and verify that everything is happening as it should: this is best done on the neighbor's lawn...
The running oil PSI on those engines is about 7 PSI with 15w40 oil at 2000 RPM, hot. At 3500 RPM it can reach 20 PSI with 20w50 oils. Don't use high-detergent (automotive) oils for sure, as the cams will soon dissolve away from the foaming detergent and resulting poor lubrication. Use motorcycle oils (NOT those labelled "V-Twin", however - that's Harley stuff).
Finally: if you want to check out if it pumps at all, remove the sparkplugs and jumper to a car battery so you can have some cranking time. Crank the engine in 5-7 second bursts, letting it rest about 10-30 seconds between tries, to lift some oil up to the top. If it's working, it will start to show up. I used to remove the exhaust rocker cover to watch for this job, but the exhaust side gets oil before the intake sides do, the right intake rocker being the last to get it.
