You make a damn good point. How will you ever know if that isn't happening for sure? Even with existing members?
And... how long must it be before you feel safe with a new member? Is it time? Or is it that they freely bitch about Sage themselves?
I caution against both qualifiers. Without other factors, time means nothing in regards to loyalty to a group. And just because someone bitches about a Sage person or policy doesn't mean that they won't by spreading information back to Sage when others do the same.
My 2 cents.
So... how will you know? One route is a more formal membership application that includes a confidentiality / non-disclosure.