Doesn't it require eye contact? Eye contact is mutual. And it appears to be automatic, not requiring an action.
As such, Sorceror A would know that Sorceror B is a sorceror, and vice versa. However, Sorceror A would then also know that Sorceror B knows that Sorceror A is a sorceror, and vice versa. And consequently Sorceror A would know that Sorceror B knows that Sorceror A knows that Sorceror B is a sorceror.
It's a moment of inescapable mutual honesty.