Of course, any other would say hitting someone or tying 'em down is pretty abusive, but leave it to us to complicate matters.

Simple answer I guess is, in the worst cases, you can't tell. This is because in the worst cases, someone who is abused doesn't think so him/herself. These will be people who think badly of themselves and are generally not comfortable with who they are. It's easy to abuse that. It's downright impossible to stop from the outside: they will find someone to hurt 'em eventually.

In other cases, such as rape, it is hard also. Noone is willing to admit to being raped, especially if that person is in the BDSM scene.

My best guess as how to guard a scene is making sure everyone knows you are the boss and where they can report problems. Also sharing information with other clubs (if any) seems a smart step to take.

If you think someone is being abused, talk to them, preferable seperate. If they say they're not, even though you think they are, you can't help. Just make sure they know where to come if and when they change their mind.