Where can i find knock sensor on elantra 2009 2.0 litre engine and how to test it?

Here's a pic of its connector's location -- follow the harness and you'll find this sensor on the right hand side of the cylinder block below the intake manifold (14mm bolt attaches it to the block).

This sensor can only be checked, not tested. If you have a DTC set (i.e. P0326 Knock Sensor 1 Circuit Range/Performance (Bank 1 or Single Sensor) the diagnostic performed by the engine computer is better than any bench test you could perform without an oscilliscope. It has a resistance you could check, but it wouldn't be revealing/absolute.