I've done a wee bit of wiring over the years but I still don't assume that just because I know how to do it that I havn't made a little boo-boo
I always do a quick check with one of these, a quick plug in, two ambers and I know I'd good to go.
The top unit has a black button on top. Install a GFCI? tap the black button and the GFCI should trip off.
The bottom unit was delibertly pushed sideways and the one amber lights
shows "no neutral" If the center amber was only on it would indicate "no ground". The codes for what light
means what is right on top, no learning required.
These testers are only a few dollars and I have one home and one in the trailer. Worth having.