Sometimes when playing a trumpet/flugelhorn part on my own, I can't tell if I'm hitting the right partial, usually in the upper register. Sometimes it's too high, other times too low. What is the proper technique to guarantee that I'm playing the right partial?
When I first started playing, I had much of the same problem as you. In fact, for the first month, I played every last partial wrong and I sounded terrible. However, I started going to the lowest partial I could hit (low C). I counted up from there. I had to do this before every piece for a while to "find my place". After that, I knew by listening whether I hit the right partial or not. However, although I could not yet always hit the right partial the first time, I did know if I hit the right one.
However, playing a brass instrument is as much mental as it is physical. You must know and be envisioning in your head what you are going to play in order to play it right. This came easier first with the lower partials and eventually with the upper partials. I would categorize my progress into four steps:
Each of these took a while. It will vary by the player, but it took me a month or two to develop my ear and to know if I struck the right note. Being able to hit any note took much longer. Most of the time was spent working on the upper register, however. It takes a while, but developing one's ear and being able to envision what you are about to play in your mind are the two most important things you'll need.
I achieved the first by counting. I would count up to a partial. Let's say E. When I sounded it, I would try to remember it and envision it. I would then play a few bars of some other piece and come back to it. I would envision the E, and hit as close as I could. If I could not tell if I hit it, I counted down the partials until I reached low C. If you have a tuner, you can just take a look at that to see if you hit the right one, but I prefer counting partials. By doing this every time I practiced for a while, I not only built up my range, but I also developed the ability to hit any partial in my comfortable range (about to the high C at the time).
The upper register uses the same concept as the middle register. The only difference is that the upper register takes more time to master. If the piece you're working on has multiple trumpet parts, play the first (or solo if there is one) trumpet part. If you can't play it all, just play the parts you are able to play for now. This allows you to get a feel for the rhythm, and the pitch. When you're comfortable with that, Try playing the second trumpet part. Typically, it's more harmony and more repetition of notes. Next, try playing it up a step. If its in F Major, move it up half a step and play it in F# Major. This will firstly, help you with transposition. Secondly, (and more importantly) you'l learn to envision what you are going to play and play it. Keep moving it up a step until you reach the top of your range. This exercise helps with many areas of your playing, but here focus on hitting the right notes just by envisioning them. It will come with practice.
This will not come like turning a light bulb on. It takes time, dedication, and focus. However, if you can envision what you want to hit, and try to hit that partial, you'll eventually get it.
If you know how the part is supposed to sound (either because you've heard it before, or because you can read music sufficiently well to hear the melody in your head), then you can obviously notice when the sound you play does not match. It can be a problem if you are playing in a very loud ensemble and can't really hear yourself (think 3rd trumpet in a big band's fff).
This helps you tell if you are hitting right or not, but you realize that you played wrong only after having already started to play the tone. Bummer.
After a lot of practice, your lips' muscle memory just knows the right feeling for a given tone and loudness. Unfortunately this changes with instrument and mouthpiece. And possibly in what shape you are that day.
If you are in a gig situation and are afraid to miss that high Ab start tone, it helps to sing it in your head before. But how do you know the pitch? You can play a low Ab softly, but that's not very discrete and does not sound professional at all. What you can do instead is blowing air through your trumpet (no sound) then suddenly throw your tongue to the bottom of the mouthpiece. Depending on which valves are pressed, you can hear the lowest tone with that fingering (that is low C down to low F#). Try to do this tongue trick while fingering C to F# up and down, and you will understand what I mean.
I would say you need both hearing and feeling. If you are practicing and still do not know if you played the correct notes or not, I'd consider: