use of coalesce to check for all variables being null
As long as all the variables involved are of datatypes compatible with sql_variant (they are in this case - basically no LOB datatypes, CLR types, or user defined datatypes) then you can use

SELECT CASE WHEN COALESCE(
    CAST(@StartDate as sql_variant), 
    @EndDate, 
    @StateCode, 
    @CountyCode, 
    @ProducerName, 
    @TaxID, 
    @Farm) IS NULL 
THEN 'yes' ELSE 'no' END


It is only necessary to cast one of the arguments as COALESCE


  Returns the data type of expression with the highest data type
  precedence.


And sql_variant has a very high datatype precedence (only beaten by user-defined data types and they would prevent the method from working anyway).

Personally I don't find this any more understandable then just finding the conjunction of the individual IS NULL results though.