Great replies. Thanks.
[My internet disconnected while starting this post. So now for my own "method".]
My criteria are build quality, tone, ease of play. Build quality is by visual and tactile. The horn must be comfortable in my hands and balanced in my arms. All parts must be solid and fit/operate perfectly.
For tone, I test with a Bach 3C. I find the 3C gives me a good balance for tone. If core, sparkle, etc are present with a 3C, I know what to expect for a 1.5C and a shallower "lead" m/p.
I start by playing low C, D, E. I am determining the intonation of the D. Next is upper E, F, G, A, Bb. If I must use excessive slide to attain intonation, I reject the horn. Then comes a Bb scale from bottom to top, again for intonation.
Then comes Clarke 2, 47, 48, 49. If the valves are not to my liking, I reject.
Then comes my own variation of Schlossberg 28, 34. I am determining how easy or difficult is the horn to play intervals and range. I focus on the harmonics of mid C, to E, to G, to the harmonic Bb, then upper C. This is an acid test. Only very good horns will permit easily the slur to upper C. Then comes lip trills. If the horn resists, I reject.
Lastly, I will play a tune in the key of Bb, as I find a Bb horn sounds best in this key. Usually it's my own version of Impossible Dream, starting on low D. I will start 'pp", and then work to "fffff" to determine whether consistent tonal quality and intonation, and whether the horn will permit me to develop full power.
This permits a quick assessment. If passed, the horn goes on my wish list.