I believe you are looking for Simulink.findVars. To find all the direct references to variables from the base workspace, try something like
varUse = Simulink.findVars(myModel, 'SourceType', 'base workspace')
To find the base workspace variables that aren't used in the model, and to avoid re-compiling the model after your last call to Simulink.findVars, try
unused = Simulink.findVars(myModel, 'FindUsedVars', false, 'SourceType', 'base workspace', 'SearchMethod', 'cached')
If you are looking for direct references to un-structured parameters, this should do the trick. To follow those references through Simulink masks or to decompose them when the parameters have structure values is a much more tricky problem, but maybe not one we need to tackle today. Is it?