How do you determine which cassette lockring tool you need?
In general you will need to know the brand of the hub and whether it's a freewheel of free hub

This page may help as a start:  http://www.parktool.com/blog/repair-help/cassette-and-freewheel-removal-and-installation 

Don't forget if it's a free hub you will need a chain whip tool as well as the correct cassette lock ring tool

Might be worth removing the rear wheel and taking it to your local bike shop if you're really not sure