You can use a template operator is_set( target )
where target = ezhttp('var','get'); and var is name of your get variable from url
see more:
http://ez.no/doc/ez_publish/technical_manual/3_7/reference/template_operators/variable_and_type_handling/get_type
http://ez.no/doc/ez_publish/technical_manual/3_7/reference/template_operators/variable_and_type_handling/is_set http://ez.no/doc/ez_publish/technical_manual/3_7/reference/template_operators/variable_and_type_handling/is_unset