There is the SPARQL 1.1 Service Description W3 recommendation but unfortunately it is not well supported yet.
You can find more in Section 2 Endpoint Descriptions (page 3) of the fairly recent paper
Buil-Aranda, Carlos, et al. "SPARQL Web-Querying Infrastructure: Ready for Action?." The Semantic Web–ISWC 2013. Springer Berlin Heidelberg, 2013. 277-293.
It is in fact almost exactly about what you seem to be doing so you might find more useful information there.
Otherwise, as @joshua-taylor says, there are some possibilities but most likely you will have to rely on triplestore-specific features. Unfortunately, not even the triple store is always identifiable via the Service Description Server
field.