No rocket science and guessing here. Remove spark plugs, remove injector harness (plugs) -- key-on to build more pressure to the rail (shutted down it still pressure in the rail but not too much as fuel pump working), look into cylinder with flashlight, crank engine keep looking if its dry. If youll see how pistons become wet -- you have to check orings (first order) and\or stuck injector. Advanced but little bit more complicated test -- remove fuel rail from manifold but do not remove injectors and fuel lines for now. Easy on s13 bit harder on s14. Ignition on -- pressure will build up and you will see what injector leaking. If none found -- put wire between 30-87 contacts of fuel pump relay to turn in on continiously -- make a button with two wires to injector connected to some 12V (any polarity). BE CAREFUL. Then open each injector for 30 secs with injector tip hanged to some bottle -- you can check actual (well.. almost) spray pattern and fuel flow (30sec flow x 2). With some quick touching battery you can check if it stuck open. BE CAREFUL AGAIN. Better make long wires and button to avoid sparks.
https://www.youtube.com/watch?v=Ddnl0pSKx9A its not mine but shows the idea, but keep it connected to factory fuel lines to save a time, just raise it and put a bottle to catch fuel.