After a lot of (very tedious) research and experimentation I've settled on a series of three MWS API calls to determine whether a product is an Amazon Marketplace Listing or an Amazon Marketplace Offering.
GetCompetitivePricingForSKU
-- This call returns an XML CompetitivePrice node containing a belongsToRequester attribute set to "true" or "false". The caveat (and the reason why I'm using three different MWS API calls) is this call fails miserably for merchants that have predetermined shipping charges for their products.
GetMyPriceForSKU
-- This call will return an error if the merchant doesn't "own" the Marketplace Listing. Purely anecdotal and empirical, though.
GetMyPriceForASIN
-- The least reliable call of these three. Sometimes it will return an XML MerchantSKU node... and sometimes it won't.
Since Amazon doesn't provide any definitive answer (or documentation) for this issue, please take all of this advice with a large grain of salt. Run your own use-cases and see which one(s) work for you.