On our first ride back after having a broken rear spoke replaced, I was struggling to get a look at the rear wheel as we rode to make sure that the wheel was remaining true. On a rim brake equipped bike, the rider gets a decent warning as an untrue wheel promptly rubs the brake pads. With a disc brake only, a wheel can get into pretty bad shape before it would become noticeable. If the rear wheel was more visible to me, the captain, it wouldn't be a big deal. Short of asking the stoker to keep an occassional eye on the rear wheel, how do people monitor this sort of thing?