How do I tell if a weapon causes void damage?

I have to kill 500 guardians with void damage. I have already tried all available weapons; How do I tell if a weapon causes void damage?

Void weapons can be distinguished by purple spiral mark next to attack value (which is also purple in color).

You can see it when you hover over the weapon or compare (arc rocket launcher on the left, void on the right):

Or in weapon details:

It's pretty easy to find such weapons on sale from Tower vendors or get as a drop from engram. Weapons can have elemental damage starting from Rare (blue) quality and up, but typically only secondary and heavy weapons have these traits. Elemental primaries are harder to find and drop mostly from completing raid activities (on the image above you can see auto rifle that drops in Vault of Glass raid).

Other damage type except from plain normal kinetic are Solar and Arc . All said above is true for them as well. For more details on damage types you can check related wiki page.

Most weapons which do elemental damage have the type they do randomly generated when they're decrypted or the merchant's stock is refreshed so it's not really practical to just give you a list of weapons. However you can tell yourself quite easily. Void, solar, and arc damage each have a special logo which will show next to the weapon. You can also inspect the weapon's upgrades, the leftmost circle will say which type of damage it causes. When you're rank 3 with any faction they will sell you legendary weapons, the special and heavy weapons they offer will do some elemental damage.

You can also do void damage with your abilities if you use a Voidwalker Warlock or Defender Titan.

Also I believe that the bounty you're referring to is actually for getting 500 "points" by killing Guardians with void damage. If I remember correctly you earn 10 points for killing a Guardian using a void subclass, 5 for killing other Guardians, and -2 when you die.