Do you have a specific 3S-FE pinout question? Leave the year and model in the comments below (or on your favorite forum), but always verify community advice against a factory PDF.
Multimeter (with continuity beeper), 12V test light, a notebook, and an FSM for resistance values. 3s-fe ecu pinout pdf
Use a noid light or a low-power LED. Connect it between Battery + and the suspected injector pin (e.g., #10). Crank the engine. If the LED flashes, you found Injector 1. Do you have a specific 3S-FE pinout question
Draw the connectors. Count the pins. Note that Toyota ECUs often have missing pins—this is normal. 12V test light