Efficient way to know Actual caller

There are two API requestSession() and releaseSession()
Both call response will be received on OnSessionChanges. Can one suggest an Efficient way to know either response is for requestSession or releaseSession.

This is asynchronous call and there is the changes that Even requestSession() is called first but releaseSession() response may come early than requestSession().

API Request
1.requestSession(ID)
2.releaseSession(ID)

OnSessionChanges(const session& sess_)
{

}