If PPP negotiations fails (ie. due to problems with authentication or lack of account on RADIUS server) session stays in transient state for some time. You can list those session

bras# show pppoe summary                                             
    PTA  : Locally terminated sessions
    FWDED: Forwarded sessions
    TRANS: All other sessions (in transient state)

                                TOTAL     PTA   FWDED   TRANS
TOTAL                            5678    5673       0       5
GigabitEthernet1/0/0             3863    3862       0       1
GigabitEthernet1/1/0             1815    1811       0       4

It’s possible to identify MAC address for modems that couldn’t establish sessions

bras# sh pppoe ses | i LCP   
   3095  28818  000e.5499.722d  Gi1/0/0.525              1  N/A        LCP  
   3548  28669  001f.a45c.7a4d  Gi1/1/0.527              1  N/A        LCP  
  13370  28817  0013.3319.316d  Gi1/1/0.527              1  N/A        LCP  
   6372  28813  0013.3199.751e  Gi1/1/0.527              1  N/A        LCP  
   6018  28812  0019.c8a3.3d66  Gi1/1/0.1598             3  N/A        LCP