I remember when I was a teenage beginner flutist I practiced constantly so that I could develop some kind of technique and some kind of credibility in the highly-competitive flute world of greater Boston in the 1970s. Perhaps once a week I would ask my little brother if he thought I had improved. I know that it annoyed him because he kind of turned my question into a pitch-and-rhythm only closed-mouthed hum, something that was idiosyncratic to our family. (We MUST go BACK the WAY we CAME became a series of sixteenth notes and dotted eighth notes--a pickup sixteenth note followed by four beats dotted-eighth, sixteenth.)
hum (ascend a fifth) HUM (back down a fifth) hum (up a major third) HUM?
Anyway, all my brother could comment on was that I might have changed my order of scales and arpeggios (and he did so gleefully). He couldn't see or hear improvement any more than I could see or hear improvement. It is kind of like watching grass grow.
Technique on the flute is all internal, so it is very difficult to notice improvement--even in yourself. There is the flexibility of the tongue which gives you the ability to double-tongue or triple-tongue quickly, there is agility of the fingers which gives you the ability to get from one note to the next quickly, and there is the whole internal breathing mechanism, which gives you quality and quantity of sound. Flutists need to be able to keep the breathing mechanism open and functional while the fingers and tongue juggle sharps in all kinds of rhythmic situations, through a whole range of intervals. Doing all this cleanly, in rhythm, with a good sound, and at the right time is a great accomplishment, but it is not one that the non-flutist outside observer can see.
There are subjective measuring sticks though, and they come up as surprises. The difference between not being able to play a piece and being able to play it comes sometimes from the long-term acquisition of technique rather than beating the piece over the head with a stick (or a bow, as I now tend to do).
String players are luckier than flutists, because they wear their technique on the outside. The ability, for example, to actually draw a straight bow solves a great number of technical problems in countless musical passages. The acquisition of the technique necessary to draw a straight bow is hard won, but it is easily seen. The most satisfying violin playing looks as good as it sounds: consider David Oistrakh's bow arm. Simple as it may seem to the non-string-player, it is a great accomplishment to have "straight" become a "default" bow setting rather than something that always has to be fixed. It requires a great deal of thought and practice.
Another measurement of improvement for string players is being able to shift accurately and correctly, and knowing exactly what are going to do once you get to your destination on the fingerboard. Most string players know their usual positions: violinists and violists spend most of their time in first, third, or fifth position, and cellists like to spend their time in first and fourth. The ability to be in second, second, fourth, or sixth position in a key with a healthy helping of sharps or flats, being able to vibrate, and knowing where you are and where you are going on all four strings is a great measure of improvement. Not having to think and look both ways before crossing and having those positions in your "default" setting, just like a having straight bow, is a great accomplishment.
Everyone looks to the outside for affirmation, but ultimately the greatest satisfaction comes from inside. Being able to play in tune is its own reward (and it has everything to do with bowing, shifting, and, vibrating). The people who are listening to you play expect it, just like they expect their soup to be warm and their ice-cream to be cold. Even the most unschooled ear can hear when something is out of tune, but most people will not comment about something being in tune. In tune and in rhythm are the "default" settings for audiences' expectations. It is after that point that the music can reach them and do what it needs to do. That's why we practice.