Almost all modern avionics systems for ether certified or Experimental aircraft use mil-spec D-sub connectors with gold plated contacts. With rare exceptions, most systems use just three types of contacts. High-density pins are typically used on IFR navigators and audio panels. Standard density sockets and pins are used on most other devices. If you are staring at a smaller number of pins in the connector—typically nine, 15, 25 or 37—those are standard density.
When it comes time to troubleshoot your system and wring out some wires, you will need to find a good way to make contact between the multimeter test leads and connector contacts. One method is to insert some spare pins of the opposite gender into the connector pins you want to probe, and touch the test leads to these “pin extensions.” This works well, but you still have to carefully hold the multimeter probes onto those pins, which can be difficult in many aircraft locations. Here are some better solutions that keep you from wishing you had three hands or smaller fingers.