GFCI position in the line
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.