Greetings Steve:
Probably the most economical way to tell if this "substance" is indeed tree sap is to simply determine if it has a sticky texture; though, this is certainly not a fool-proof test. If it is not sticky, you'll likely have to examine the building materials more closely.
Although I would have to research tree sap volitalization more thoroughly, the majority of "sap" spray (i.e., honeydew) is actually from insects (aphids and scales). This is a common complaint among vehicle owners who park adjacent to or beneath the crown radius of a tree.