From the application's perspective, there is no way to distinguish a user's application source from the proxy source on a UM store.
> Alternatively, is there a api which I can use to find the last sequence ID for a given source ID?
In general no. The problem is which last sequence number where? The only sequence number available to the application layer is the topic sequence of the last message the application has received.
There are a variety of sequence numbers: at the topic source, at the UM store, and in the receiver, all of which are changing.