Do you have any off-leash dog parks that are fenced in? You can test her there first. Does she have a strong recall (comes when you call her)? If she doesn't come straight away after you call her, you might want to work on recall before you think about letting her off the leash in an area that isn't fenced.

My advice: If she has a good recall and usually stays near you (isn't one of those wild, hyper dogs that are uncontrollable and like to run off and stuff) and follows Rebel (which would be good since you said he listens to you), then maybe you can test her off the leash in an open field away from traffic and possible distractions (such as cats and squirrels). Or maybe keep her on the leash the first time, but just drop the leash.