There is an instrument that no one has mentioned yet, marketed by GageMaker. Essentially, the simple version uses a contact ball fixed on one end of an adjustable "C" clamp shaped rig with a 1" dial indicator on an adjustable arm. The indicator also has a ball mounted in the stem. They offer software to calculate the setting distance between the two balls to the exact PD to the basic PD, then you read the variation, either plus or minus from where it is set. The software compensates for the helix error, and can be adjusted for different ball sizes, in case you don't have the recommended ball size. Just like wires, there is an ideal ball size that contacts the flanks close to the pitch line. They offer contact balls for Acme and Stub Acme with precision ground flats, since the 29 deg flank angle would allow the ball to hit the root, or minor diameter before it made contact with the flank at the pitch line. This is also offered in an internal version.
Additionally, they make a similar instrument that used what they call "rolls" instead of balls that contact several threads at once. Helps detect lead error instead of a separate lead gage,which they also offer.
All in all, it's a little awkward a system, but if you don't have a ring or plug gage, it's better than nothing. One advantage is that you can measure many,many threads with the system and not have to have a huge gage inventory. They also offer an instrument for working with tapered threads as found on API tool joints.
Oh, and it's rather expensive, so not likely practical for the hobbyist.