Hi Tim,
I'm sure you meanwhile found the answer - but maybe this helps other users finding your posting to solve their problem (as I also spent some time to get the information)
I found a kernel function LOGIN_INFO in BADI CL_SSFT_SYSTEM_SIGNATURE providing this Information ..
* Determine how user has logged on: P=Password, S=SNC, X=X.509, E=External CALL 'LOGIN_INFO' ID 'OPCODE' FIELD co_session_info ID 'AUTH_TYPE' FIELD rf_auth_type. "#EC CI_CCALL IF sy-subrc <> 0. CLEAR rf_auth_type. "unknown how user has logged on ENDIF.
If the value of rf_auth_type is 'S' the logon is SNC + SSO ... If you log on with UserID/Password and SNC nwtwork encryption the value is 'P'.
Best Regards
Christoph