I bought one of them Snap Safe hot rods, same concept like the Golden Rod or MAX Safes rod. Only problem is, after I removed the wall outlet from the cable to feed it through the small hole in my safe, I can't remember the orientation of the polarization that I put the cap back onto the cable (i.e., neutral or line out ends on the outlet plug cap in relation to the cable's 2 lines).

How would you know if the rod is working correctly, in terms of the temperature it should be? If it was improperly connected, one might expect a different temperature indicating that a resistive element of some kind is burnt out, or lukewarm/ambient room temperature instead upon touching it...

I prefer a PEET Safe Dryer. It works better than a Golden Rod.

If using a Golden Rod, simply touch it and see if it’s warm.

Should be warm to the touch but cool enough to handle.  I got the temp meter out to see how mine was and it averaged about 95 degrees.  
I have been using the same goldenrod for 20+ years, the only downside is you have to position your safe close enough to an electrical outlet. it is a pretty simple resistance rod, the polarity should not matter.

It is AC, but if it doesn't have a ground and isn't double insulated, you don't want to touch it with another similar device that is wired correctly, if you have it wrong.

If you have a meter, set it on AC and put one lead in your ground plug and then measure the outside of your heater to make sure it is not at line voltage.