My first paper as an undergrad measured MIDI round-trip by tapping the input and output MIDI wires and then looking at how the signals were offset: https://www.kmjn.org/publications/MIDI_ICMC04-abstract.html

For keyboard input, one paper (whose citation I can't seem to find) used a clicky mechanical keyboard, pointed a mic at the computer, and measured the offset between each corresponding pair of keyboard click and sound output.