I am installing a GFCI. There are several other outlets in the same line. How can I find out which of the other outlets is downstream from the GFCI?
First ascertain the complete set of outlets that are on the circuit by testing which ones go off when the main breaker is turned off.
Now turn the breaker back on and then press the TEST button on the GFCI. The result of this is that all the outlets that are connected to the LOAD terminals of the GFCI will go off. Those that are up still powered are those that are upstream from the GFCI.
Get a bunch of dollar store night lights, the kind without an automatic light sensor. if it has a switch, turn the switch on. You can use any other device you may already have, as long as it's easy to tell if it's on or off.
Turn off the circuit at the breaker. Stick a nightlight in every receptacle. If the nightlight turns on, that receptacle is not on that circuit.
Turn that circuit back on at the breaker. All the nightlights should be on. If any are not, you have a separate problem.
Now take a guess at which is the receptacle "electrically nearest" the panel. Shut off the breaker and disassemble the receptacle, unhooking one "hot" wire and capping it for safety. Put the nightlight back in it. Turn the power back on.
If you guessed right, all the nightlights are now off, except maybe at this location. If this light is on, the hot wire still connected and its neutral are LINE. If it's off, they are LOAD.
If you guessed wrong and a light is still on, then the first one in the chain must be one of the lights still on. Guess again.