Just in case you aren’t familiar with the lingo yet, BIS is the Integration Server Built-in Services guide. In that guide, look for pub.publish:envelope. The pubId field is likely what you want.
Just be careful with how you use this information because, as you are probably aware, one of the key advantages of a pub/sub solution is the decoupling of publishers and subscribers. If you have code on the subscriber side that is specific to certain publishers, it kind of defeats the purpose.