Neon bulb tester- Is the voltage AC or DC?

So are florescence bulbs next to a transmitter.
Then there is the Gound Fault Interrupter testers. And Arc fault interrupters.
I've been to a few places online and some say there not reliable. Most used Neons at one time, now there L.E.D. lights .

Many of us have those old neon bulb testers used to see if an outlet has power.
 They generally used a NE2 bulb.

Now if both electrodes light up- it is an AC voltage.
If only 1 electrode in the lamp lights up, its DC.

Quick and dirty test.

Also good for testing for the presence of RF.