How safe are voltage detector pens and how can I test one's safety?
That's what I'd call a neon screwdriver, it has a neon indicator that lights by mains voltages starting at around 70V 

it's main advantage over the non-contact voltage detectors are that it's always ready because it does not have batteries that can go flat. 

its disadvantage is that it is less sensitive and requires physical contact with the conductor to operate.  the one I have here is stamped 100-500V 

less than 100V you don't get reliable detection, past 500V there's a risk of electric shock if the insulation fails (ther insulation internal to the resistor is a point likely to fail if the safe voltage is exceeded).



Non-contact voltage detectors on the other hand sense the electric field in the proximity of live wires. it can detect voltage therough the insulation on the cable and its plastic tip. Its nternal electrionics amplifies the signal so it gives a stronger visual indiccation and a audio indication too.  the one I have is rated 50 to 1000V, so not only is it more sensitive, but also it has better insulation, requires two AAA cells which last a long time.

For safe testing, test it on an outlet that's protected by an ELCB / RCD. with such a set-up  a failure could be unplesant, but not life-threatening.