Again, not knowing the Mustang, but typically there are good cues. If the landing gear cycles correctly while retaining a single bad indication, it moves the bets towards an indication problem. If the landing gear is symmetrically, and rigidly, in correct position, and doesn't move around, it probably is locked. An exception to this is mechanically-driven landing gear, typically using screw-drive: Piper Comanche, Mitsubishi MU-2, and Focke-Wulf Fw 190 come to mind quickly, but these generally don't collapse as readily, if just a bit short of "locked" position (if actual locks are used at all). A hydraulic landing gear tends to move around if it wasn't locked in place. You mention the doors; in cases mechanically actuated sequencing valves are used to close the doors when the gear reaches some given position; these may or may not give some additional info of the situation.MarcE wrote:if the bay doors require a locked landing gear to close, ok... but otherwise, how could that be seen?
Further, in many airplanes, the downlock condition is achieved by the geometry of sidebrace. This is more or less visible from the ground. In case of P-51, it appears that the landing gear downlock is achieved by a spring-loaded pin that locks the pivot shaft arm. This would not be directly visible from ground most certainly, so one would need to make a guess based on how solid the leg seems.