That's right. Use ohmeter, check "field to field" should show quite low resistance
Check both terminals to case, both should show infinity
There are rare cases where a rotor can short internally to the "shaft" and this can cause this conditon, but rare. By far the more common cause is brush / brush holder problems........mistakes in hardware by rebuilders, MODIFIED (round back modified to isolated field) by rebuilders, or mistakes in owner assembly, broken or missing brush holder parts
