Web Service Call Authorization

We are creating a new procedure where all our external web service calls to external vendors need to be authorized (the user account used to make the call is part of a specific group).

 

How would i pick up the username being sent in the service call? and if the username is part of the call, do i write an irule to confirm that or would i use APM to run an LDAP query against that user?

 

Ideas?