2001 Chevy S-10 my engine will not start

My engine will not start. It has central multiport fuel injection. To determine if it was electrical or fuel related. I put a small amout of gas in the throttle body. It starts that way and will idle but dies when I give it any throttle. I replaced the fuel filter and I can hear the fuel pump run. I checked the fuel pressure and it is low. It should be between 55 and 61 psi when not running and the key on. I have measured between 45 and 55. Typically the 45 to 50. It will hold the pressure so I don't think an injector is leaking. Ive been told it is probably the fuel pump. Is there a way to determine if I have a bad fuel pump or pressure regulater (both are hard to get to). I would like to figure out which one is the issue before removing the tank or air intake plenum (thanks for any help)