One topic that comes up often is that of testing new and used horns, a good topic and one I have not written about in any depth.
Before I get to my thoughts though, there is a two part article by Bruce Hembd on the topic in Horn Matters, it starts here and is well worth a look as well.
Over years of trying horns at shows and with students, I have a few things I check, more or less in this order.
How does it look? Some horns are better built than others. An expensive horn should give all appearance of being built very well in terms of fit and finish. If it is expensive you should not see obvious things wrong.
Pick it up and look more. How does it feel in the hands? Even before you play one note, it should feel balanced and ergonomic. The valve throws should be easy and quick — especially so if it is an expensive horn.
The next step is put your mouthpiece in. I usually carry a couple, one with a standard shank and one with a European shank. Look at the fit. There is a typical and correct distance it should fit. If it does not fit correctly the horn won’t play at peak for you.
How does it play? You will want to play something right away but the first stop needs to be a quick and basic check of tuning. I have some basics on this topic in the article “Putting the Slides Right.” The critical factors at this point are understanding what the slides do on the horn in your hands and how to balance the F and Bb sides. If they are not in tune with each other with the valve slides at some roughly accurate place all the rest of your testing to come is to a point a waste of time.
Then actually play the horn. Test slurs and articulations with some trills and some loud short notes in the middle register and then get to some critical notes. Testing horns at workshops it seems a lot of people play a lot of high C’s, but actually they should be testing high Bb. This tends to be the one critical note (more on this topic here), especially on Geyer style horns. If a horn does not have a high Bb, you may be able to help it if you love everything else or it is a great deal or whatever, but on the whole you are probably better to just walk away. Also if the low range does not speak well, you will just get frustrated playing the horn over time, walk away.
If you are at a show and can AB test a number of horns another thing interesting to try is play some slurred scales or slurred passages and observe the relative level of “clicks in valve changes.” More on the topic here, it is an element that is actually pretty easy to objectively observe in this context, some horns will be ultra smooth but many are not.
Finally, you need to try the sound out in a big room and get some good ears listening to it. How does it really sound? Would it work in a large orchestra or is it a bit under powered? Etc. You can get some sense of this if you focus your attention on the sound out in the room instead of the sound right at the bell.
And I did not mention the topic of “how it feels” yet. This is the one thing that really will depend on where you are coming from. If you have been playing a lower end student horn a long time then almost any professional horn will sound and feel great, some better than others of course. But I will say that when I was playing my big Paxman 25A that I would try horns at shows and basically every horn to a point felt stuffy. Now that I am not playing it regularly and purchased a Geyer style horn, lots of Geyer style horns feel great to me. So do keep that in mind, what any pro tells you will be colored by how a horn feels in relation to their own primary instrument.
It is a difficult quest, the search for a great horn. And you certainly need one! Good luck!