Check to see if an object (lookup table) exists

There are two ways to do this.
The first is to use Exception Handling, and catch the Exception.
Once that has been done you can use methods like GetBaseException() and test if it was caused by an item not being found. Here is a code example that tries to perform as TransformText on a non-existent RuleGroupName (Lookup table name), but *without* bombing out with an error:

'--------------
Dim strRuleGroupName As String = "E101_Acct_Description"

Dim strResult As String = String.Empty
Dim bTableExists As Boolean
Try
  strResult = brapi.Utilities.TransformText(si, "11000", strRuleGroupName, True)
  bTableExists = True
Catch ex As XFException
  Dim baseException = ex.GetBaseException()
  brapi.ErrorLog.LogMessage(si,"Inside exception, ex.Message:" & baseException.Message)

  If baseException.Message.Equals(GeneralStrings.ItemWasNotFound) Then
    bTableExists = False
  Else
    Throw ErrorHandler.LogWrite(si, New XFException(si, ex))
  End If
End Try
brapi.ErrorLog.LogMessage(si, "Lookup table " & strRuleGroupName & " exists? : " & bTableExists.ToString() )
'-----------------------------

The example above checks the originating Exception is in fact caused by the lookup table not existing, by comparing with the pre-defined message constant : GeneralStrings.ItemWasNotFound , which would be thrown deep inside one of the internal WCF classes. This way if another type of exception occurred (which has nothing to do with the lookup table not existing) then it would generally throw an error otherwise, which is what you probably want.  Basically, when I catch an exceptions never assume the cause of it, hence the test on the BaseException's cause.
And *obviously* take out the LogMessage statements when you are done testing!

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 ).

Thanks!  I had thought about the exception but was hoping for something more preemptive.  That should work with any set of objects though.  I'll probably try both and see which works better here.  

Just a heads-up, if anyone's thinking of using
  brapi.Database.GetCustomDataTable( )
... and then just passing it the name of a standard XF table, the function actually checks if the table name you passed is a standard table or not, and just returns (nothing) if it is a standard XF table.  So you'd either have to use unsupported internal object methods , or regular SQL (and cache the resulting DataTable if calling repeatedly).