I am slightly surprised that no-one has mentioned the basic technique that I was taught.
Keeping the VOR at 90 degress, plus the DME indicating the correct distance are the primary outcomes of a correct arc, but the performance indication that you need to actually look at is the DME speed (which is the rate of change of distance). If you are maintaining the arc, the speed will be zero. If you are off the arc, the speed will start to creep up - simply turn in (or out) as required to put it back to zero.