A testing sequence that has proven efficient includes:
Play something familiar on the harp to get an overall impression. Both harpists should do this, and perceptions from both the playing position and the audience position discussed.
Check the instrument for even sound by playing each string in turn from bottom to top. Then play chords in the top, midrange, and bass to be certain that the sound is even. Neither the bass nor midrange should overbalance the rest of the harp. The highest treble notes should ring clear, without the unmusical “plick” that one sometimes hears. There should be no “wolf tones”, or notes that stand out as unusually loud. A single dead sounding note is usually a bad string or a regulation issue if the adjacent notes sound good, but this should be checked.
Use an accurate tuner to re-tune the harp with all the pedals in flat position and then put all the pedals in sharp. It is usually possible to regulate the flat to natural interval accurately, so the test of a well designed harp is intonation in sharp. Check the intonation of the sharp notes through the range of the harp. Some early Wurlitzer and L&H harps are significantly sharp in the top two octaves. To get them to play in tune it is sometimes necessary to replace the neck with one designed to increase the active string length slightly. Some modern L&H harps have two or three notes in the second octave that will not regulate in sharp. For some harpists this makes the instrument unpleasant to play, but some become accustomed to the sound and ignore it.
Play something like the Bach “Prelude #1 in C” to get a perception of the clarity of the sound. Individual notes should be well defined, without a trace of muddiness. Then play something with full arpeggiated chords. The harp should sound rich and satisfying.
Play something loudly, then softly, and note the effort required and the perceived dynamic range. The greater the dynamic range and the less effort required to get a true Fortissimo the better, as long as the instrument is not flabby and muddy sounding when played loudly.
Pick the harp up and move it a few feet. One needs to be reasonably comfortable when handling the instrument, even if an assistant is usually available.
This sequence will provide an excellent idea of the capabilities of a particular harp. Examining several instruments the same way will provide a valid understanding of the relative merits and weaknesses of the harps tested.