Love those plug in ring mains testers for quick diagnostics with this sort of issue.
Prefer the uber cheap three LEDs on means all OK, like this one **LINK** .
Check in known good socket first.
Then all on means the suspicious one OK, the one or more off shows where the fault is.
Its pretty hard to get wrong.
With the clever ones where you have to read the list of light combinations to figure out exactly whats wrong its easy to make a mistake decoding things. I've been told by "one who ought to know" that there are some, theoretically never found, weird issues that can confuse them too.
Clive