I replaced a 2-prong outlet with a GFCI, wired as shown (connected ground wire coming into the box to the metal box as well as the ground screw of the GFCI, black wire to brass terminal & white to silver). The box has no separate grounding wire and wires are only in Romex, not metal conduit. Multimeter shows hot and neutral are correct. Later, I realized I never checked whether the ground wire was actually grounded at the service box. To be code compliant, do I need to apply the "no equipment ground" sticker regardless, or only if the bare copper wire coming in is not grounded at the service box? 
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Can you turn the breaker for the circuit OFF and use a multimeter to check for continuity from ground to neutral?– ThreePhaseEelCommented Apr 4, 2021 at 23:21
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Yes, but later in the day. Other people in the house are using that circuit now. A simple lighted circuit tester shows "no ground").– TucsonTerryCommented Apr 4, 2021 at 23:26
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You need to run the ground wire to the metal box first. By the way, you see the funny thing on the lower mounting screw on the GFCI? That is called a "self-grounding" feature. That means you don't need to run a ground wire to the recep, it will pick it up off the metal box.– Harper - Reinstate MonicaCommented Apr 5, 2021 at 3:11
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Thanks. When I was installing, I connected the "ground" wire from the 12/3 romex. Later, I realized I hadn't verified that the bare wire was actually grounded. A circuit tester indicates it is not. I will use the "no equipment ground" sticker to comply with code.– TucsonTerryCommented Apr 5, 2021 at 3:21
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1 Answer
The NEC in 250.130(C) now requires extending an Equipment Ground Conductor to the grounding system when making an extension of a two-wire circuit that does not have an EGC.
If your circuit was extended before the current requirement or was extended by local exception then the receptacle needs to be marked " No Equipment Ground" and the ground should be left disconnected.
406.4 General Installation Requirements (D) Replacements
406.4(D)(2) Non–Grounding-Type Receptacles. Where attachment to an equipment grounding conductor does not exist in the receptacle enclosure, the installation shall comply with (D)(2)(a), (D)(2)(b), or (D)(2)(c).
(a) A non–grounding-type receptacle(s) shall be permitted to be replaced with another non–grounding-type receptacle(s).
(b) A non–grounding-type receptacle(s) shall be permitted to be replaced with a ground-fault circuit interrupter-type of receptacle(s). These receptacles or their cover plates shall be marked “No Equipment Ground.” An equipment grounding conductor shall not be connected from the ground-fault circuit-interrupter-type receptacle to any outlet supplied from the ground-fault circuit-interrupter receptacle.
(c) A non–grounding-type receptacle(s) shall be permitted to be replaced with a grounding-type receptacle(s) where supplied through a ground-fault circuit interrupter. Where grounding-type receptacles are supplied through the ground-fault circuit interrupter, grounding-type receptacles or their cover plates shall be marked “GFCI Protected” and “No Equipment Ground,” visible after installation. An equipment grounding conductor shall not be connected between the grounding-type receptacles.
Informational Note No. 1: Some equipment or appliance manufacturers require that the branch circuit to the equipment or appliance includes an equipment grounding conductor.
Informational Note No. 2: See 250.114 for a list of a cord-and-plug- connected equipment or appliances that require an equipment grounding conductor.
answered Apr 4, 2021 at 23:49
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So, if I find the bare copper wire coming in is NOT GROUNDED at the service box, I'll disconnect that wire from the receptacle and mark the outlet "No Equipment Ground." But what if I find that it is GROUNDED at the service box? Leave the wiring as is and no sticker required? Commented Apr 5, 2021 at 0:00
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Any ungrounded 3 wire receptacle must be labelled. Something is missing if the tester shows no ground. If it is a cable in the panel that has a ground but isn't connected it needs to be corrected. A cable connected in the panel but the ground not being complete to the receptacle the circuit needs to be repaired, a GFCI is not an acceptable alternative to repairing damage or fixing an incomplete or poorly installed splice. Commented Apr 5, 2021 at 0:24
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Thanks. I wasn't sure whether to trust the simple 3-light tester. Posters elsewhere indicated GFCI's "always" show as ungrounded with these testers, but it was unclear whether they assumed two wire/ungrounded wiring. Commented Apr 5, 2021 at 0:34