Yeah, I just happened to use this yesterday. isObject(%player.getObjectMount()) works beautifully.
Edit: Wait, that only gets if you are in a vehicle. Use Marble's %vehicle.getDrivingClient().
I guess you can use if(isObject(%veh = %player.getObjectMount()) && %veh.getDrivingClient() == %player.client). You might want to check that its a vehicle and not a bot, because I'm not sure if bots have the function getDrivingClient.