To get the exact list you need to look into the source code to see what all the options are. It can change with model version and exactly how you configure your simulation. You look for the "call hist_addfld" calls in the source code.
The CLM User's Guide though does have a list of the default variables that are output for a fairly standard simulation for CLM5.0 and for the latest CTSM development version.
See
There's a drop down menu at the top to toggle between CLM5.0 and the latest development version. This listing just gives the default fields, there are actually additional fields that can be output that querying the code will allow you to fine.