There is a way to determine what proc a user actually used to signon with if a user uses the TSO full screen to change the TSO proc. When specifying the TSO proc on a TSO full screen logon ACF2 will create a GSO TSO record for that user that will contain full screen change information, including the TSOPROC.
To list the TSO record for a single logonid or all logonids the following commands can be issued from TSO, ACF:
TSO
ACF
SET CONTROL(TSO)
LIST logonid or LIST LIKE(-)