In the current alpha there's no simple way to do this, because we haven't yet added a bunch of fields to the API responses to represent stream items. Work to improve this in the next alpha is already under way.
In the meantime as a quick hack you can use the verbose option of APIs like listwallettransactions and examine the vout array. Published stream items will have two outputs, where the first has "type" : "nulldata" and contains the item. Regular sends with metadata will have three outputs, where the second one has "type" : "nulldata". Note that this is not guaranteed behavior in the long term, but we won't be changing it before there's a better way of identifying stream items in the output from listwallettransactions.
