Check to see if an object (lookup table) exists

The other option would be to lookup the StageRuleGroups directly.
You could either use plain old SQL for this (just ensure you don't open/close connections inside a loop) or the BRAPI methods to retrieve a datatable, using table name of "StageRuleGroups" and the lookup column "RuleGroupName".  If doing this in a loop across multiple entities , in an Extender Rule, then obviously you should pull all the rows into a DataTable first, before entering the loop, and then lookup the row in a Dictionary object if you need to perform lookups inside a loop aross multiple entities. You don't want to continually be running SQLs inside a loop, or on every data unit being calculated ( For Finance Business Rules then cache the DataTable in the Globals object, or use the api.Functions.GetCustomBlendDataTable() to automatically read and cache a data table , which doesn't have to be a BI Blend table despite the name of the function ).