How do you determine which cassette lockring tool you need? I'm looking to order the tool to remove the cassette lockring on my bike, but I'm not sure which tool to get.  Is there a way to tell just by looking at it, or do I have to look it up using the make and model of the cassette?



EDIT

While I was looking for a more generic answer, specifically I have a Shimano CS-HG20-9 Cassette. Seems to be a freehub style, and probably takes FR-5 tool. However, I wanted to know if there was a way to know for sure that the tool will fit, so I don't waste time ordering the wrong tool.
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