I’m also trying to figure this out and the MWS API is frustrating.[edit: I was having trouble because my php MWS library was old, make sure you’re updated] What I’ve learned is that tax facilitator information is not part of the Order, but done on a per item basis. So, I retrieve an order, and then retrieve the items, and then within each taxed item should be a TaxCollection() attribute that has the properties of “Model:MarketplaceFacilitator” and “ResponsibleParty:Amazon Services, Inc” whenever Amazon collects taxes on out-of-state sales for me.

you can see this for yourself https://mws.amazonservices.com/scratchpad/index.html
at top-left, use API Section “Orders”
and Operation “ListOrderItems”
then fill out your credentials and a provide an amazon order ID to explore the response. Within the response, you will find < TaxCollection >

so, you can use code like this:
$Items = $service->ListOrderItems($req)->getListOrderItemsResult()->getOrderItems();
$taxCollector = $Items[0]->getTaxCollection(); (or loop through all the items)
if( $taxCollector ) $facilitator = $taxCollector->getModel()

if no taxes were collected, or if the tax revenue was sent to you, then the $taxCollector will be NULL
if amazon collected and paid out of state taxes for you, then you should get a model of “MarketplaceFacilitator”