From reading the topic heading, the immediate answer seems quite easy:

Abuse is anything non-consensual

But reading the question fully, i can see your dilema Tom and yes, you are right, abuse victims with low self esteem won't say it's abuse unfortunately.

As GS42 says, try talking to them separately and give them your support and maybe they will come to trust you enough to tell you what's really going on.

Alternatively, i think you personally probably know the people and players in your community and who are the valid ones and who aren't so you may want to try to go to another submissive to talk with who you think is being abused, sometimes it's better accepted coming from a peer and not another authority figure

Really education and awareness will help - show the victim that it's not acceptable to be treated as such and any i have met in my local community are quick to share that information and to be sure everyone is being safe