There's no need to dismount the tire. A Hunter balancer can measure the rim's run-out, tell where the heavy spot on the tire is, and how much (if any) the tire should be rotated on the rim to have the two cancel each other out as much as possible.
Of course, it does require someone to put the wheel on the balancer correctly, that the balancer is properly calibrated, and that the person operating the balancer is willing to do what the machine suggests.
Finding that capable person is probably the biggest challenge.