Ballast resistance values are tested by disconnecting one or both of the wires connected to the resistor, setting the V-O-M (analog device like a Simpson 260 or Tripplett 630) to the R x 1 setting, shorting the rest lesds together, zero the instrument to show zero (0) ohms (usually full scale), unless it's an digital auto ranging instrument, then connect the rest leads to the resistor then read the resistance in ohms directly. When measuring resistance, no external voltage is applied to the device being tested.
3 1/2 or 4 1/2 digits digital auto ranging devices are extremely accurate a relatively inexpensive...like Harbor Freight's offering at less than $ 10.00.
I have never had a ballast resistor short, thus applying the full 12 volts to the coil at all times. I have had them open, start and die or not start at all. You actually need a pretty good meter to read tenths of ohms. Checking for voltage drop is easy, check on both sides of the resistor(s) to chassis ground under all evolutions. Key off, key on, starting, running.