View Variables

The easiest way would be to check out the Streamer.bot docs and look for the trigger or subaction you are using. A full list of variables will be given there, e.g. Twitch Follow trigger.

A second method would be to run the action, then go into the Action Queues tab, into the Action History and then doubleclick the action that has run. This will give you all variables that have been populated. (Example: see here)