Abusers are often clever and it can take a long time for them to be found out. Frequently it is a pattern of apparently small incidents or observations that can unmask them.
It is important that you record the details of your concerns or an incident while you can clearly remember them, including date, place, time and names. Don’t wait until something else happens because you won’t be able to recall the details accurately. Even if it is just something small that is nagging at the back of your mind, make a note of the details. It may occur again, or you may notice other things that confirm your suspicion, or other people may have also noticed something so a pattern starts to emerge.
If you seek advice or go on to report your concern then record this as well.