A magnetic dial indicator would be the contraption to which you refer. The farther out you can get from the bearing would be most effective, as a bent axle won't be apparent right at the bearing. If you brake drum is concentric, you could just adjust the brakes until they just drag and listen as the wheel is turned slowly to determine if it drags more at one point than 180 degress from that point. Why do you suspect a bent axle?