When faced with an old but working flash offered to you, and wondering whether to risk acquiring it (possibly to find out it is a true high voltage model), there is a quick test that often (not always!) gives you a hint about the kind of triggering circuit used:
High voltage designs USUALLY allow triggering (also via the test button) with the unit charged but turned off.
Designs with a low voltage (6-30V range) trigger circuit USUALLY do not. One notable exception: All Metz 45CT-1 variants (some are high voltage, some not) do.
Also, anything using an SCA300/3000 style foot (eg some Metz, Braun, Osram units) tends to be in the 6-24V range.
There are exceptions, and safety for a given camera can only decisively be found out by measuring.
Note that for some cameras, everything above 6V or even 3.3V is out of spec and unsafe.
YMMV, but the cheap kind of radio triggers, specced to 12V, often handles 24V fine (and at $5-10 a receiver you can risk it. Mind that some need a MINIMUM distance between sender and receiver!). Not surprising - there are not that many electronic components/circuits (excepting such that need a 12V supply too) that, under low current situations, can switch 12V to ground but will die from 24V. Most IC logic level outputs cannot handle 12V, while there are not many discrete transistors/thyristors that cannot handle 24V. Specifications of 3.3V, 5V, or 6V however do sound like IC outputs directly wired to the hotshoe - 12V would damage them.
Note: When handling a found flash unit of completely unknown history, it is wise to let it charge for at least a cigarette's length or two before attempting to trigger it. This will allow the capacitor to re-form a bit. Supervise it, and obviously turn it off and put it in a non-flammable place if there is any smoke except that from the cigarette. If you want to be extra careful, use partially run down alkaline batteries or a current limited lab power supply.
Obviously, really refrain from attempting to disassemble these things, especially when charged - they are cramped and illogically built inside, things can unexpectedly fall into your hands or conductive objects, and there is enough energy stored in the capacitor (and often plainly accessible once the case is off) to a) make you a doctor or coroners concern if you get shocked by it, b) make your ears ring if you accidentally create a short circuit with a tool...