Most of the people on this forum have found the online status to be very unreliable. For me personally (and I read the same opinion from another member - Payala), the online status has worked perfectly well, yet, I haven't seen any change in the online status of I-485... even the LUD didn't change. I doubt that there is a way to know that FBI check is complete.
BTW, would you mind posting your timeline and DO?
USC