my question is in the title, any help is appreciated.
If it loses power after you trigger the GFI off.
Also if you test a receptacle and it is already dead its possible that the upstream GFCI device has already been tripped. Getting to know the locations where GFCI protection is required can help you to find an upstream device.
Downstream outlets/receptacles will have no current when a GFCI outlet, or device, is tripped.
You can also trip typical outlets to either side of a GFCI outlet to see if that outlet is slaved or GFCI protected.
Just because the outlet is not identified does not mean it is not GFI protected. Bathrooms or Spa’s might have a GFCI device you are unable to readily observed.