Low pressure tub & shower faucet; kitchen sink and bath sink pressure is fine. How do I check the pressure before changing the T & S fixture?

Try this. Shut the water off and pull the cartridge, Check the inlet ports for trash and clean. Now place a pan over the open valve and turn the water back on for a minute to flush out the supply. You should have a strong stream when doing so. Reassemble and test. If that works do it on all affected taps, good luck, Tom