Those are the most dangerous tools you can get.
They rely on you completing the circuit to light up the neon light by touching the metal on the cap of the screwdriver.
simulate this circuit – Schematic created using CircuitLab
There are a lot of dangerous conditions it cannot see:
- You are not grounded, but the line is live.
- The voltage is not high enough to light the neon light. Which is probably why they aren't common in 120V regions.
- The resistor or lamp is damaged and shorted or open.
- Floating potentials (behind isolation transformer).
If you insist in using an contactless detector, buy an active one. Fluke VoltAlert for example. But read the manual and know the limits of your equipment.
But the best way is to use a meter with a low resistance mode. Like the Fluke T90, to make sure what you measure is correct, and not a "ghost voltage".