How can I tell what dimmer switch to use with LED lights?
You will need to check the product packaging or manufacture website to see what types it is listed for.  All of the Leviton and Lutron products I see at Home Depot lately have clear markings on the packaging.

Lutron marks them as C-L and Leviton calls them "Universal Dimmers".

If your lights are 12V then you need a magnetic dimmers for low voltage lighting.