On the Vortec engines (96-up), there's a schrader valve you can hook a gauge to. On small blocks, it's to the driver's side of the black square plastic "Vortec" cover. On big blocks, it's near the front of the intake between the thermostat housing and the plenum.
On your TBI engine (87-95) it's not quite as simple, but still not a big deal. You have to buy a kit that lets you T it inline by the fuel filter, in the driver's side frame rail.
For a Vortec the fuel pressure spec range is 60 to 66 psi. Vortec range - the 4.3, 5.0, 5.7, and 7.4.
For a TBI 4.3, 5.0, or 5.7 - the fuel pressure spec range is 9 to 13 psi. For a TBI 7.4, it's 26 to 32 psi - I've learned recently that for 1994-1995, the TBI 7.4 is 26 to 32, but prior to that it's the same as the small block specs.