how to know what is the enemies damage type ?

[ohmygah]

how can u guy know what is the specific type of damage like void or arc ,... from the enemies u guy r facing with

[EckyThump]

It is usually colour-coded. If it looks fiery (Taken Knights, Cabal Incendiors) then it's likely Solar. Blue and crackly, then Arc, like the melee attacks from Taken Marauders and the ranged attack from Hive Wizards.

Also, if you die to it, it will tell you what it is called and what element it is, top right of the Guardian Down screen. The 'Taken Slug Rifle' used by a 'Ravenous Taken Phalanx' will confirm that it is Arc.