That I understand.. The advice I give is to always test with meter first.
Then test again
Then when you are all done test yet again
That really is how I do it.
I also have (Somewhere in a tool box around here) a special 12 volt plug, It has a 2-lead "Tri-color" LED and resistor in it.
If you plug it in and it lights up GREEN.. you are good, you have it wired properly.
If it lights up RED. you goofed.
and if it lights up YELLOW.. You got bigger problems (That is an AC outlet)
Works great.. I built it back in the days when cars 12 volt outlets might or might not be wired properly. Simple circuit.