I've figured out how to do it for my specific case: ENVI> clusterLog = ClusterLog() ENVI> print, obj_class(List(ClusterLog)) LIST ENVI> print, obj_class([clusterLog]) CLUSTERLOGGER As well, I could use size(expression, /type) to first determine whether the argument is an object or array of objects.

You can also use the functions ISA or OBJ_ISA.

You can also use the functions ISA or OBJ_ISA. Thanks. OBJ_ISA is much better (no need to call size(expression,/type) first). ENVI> print, obj_isa(List([clusterLog]), 'LIST') 1 ENVI> print, obj_isa([clusterLog], 'LIST') 0 ENVI> print, obj_isa([], 'LIST') 0