Eventually you'll learn to tell what a nice tone is, however there are things to take into consideration:
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Solo or band play - different requirements for both, obviously. With solo your chanter just has to match itself and your drones. With band, you're matching to the rest of the band.
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Climate - temperature, humidity, altitude all have an effect on the instrument. This is a risk when people only tune to a number. Sometimes you have to tune to what your pipes will sound nice at, rather than a number you've seen online - particularly in more arid or tropical climates.
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Playing time - you'll get a different tone when you first pick up and play vs 15-20 minutes in. Don't tune straight away and then leave it - your pipes will change once you put a bit of air through them.
My general approach is this, regardless of solo or band. Start playing, and give my drones a quick tune (I've been playing long enough that I know the tone I like, and can lock in reasonably close), then I play for about 15-20 minutes with a few different tune types - airs/slow marches/waltzes to really listen to the tone, marches/jigs/strathspeys to listen to the transition in the chanter (particularly high/low). Then I'll tune the chanter.
My pipes are pretty stable, and we usually tune around 482 (due to where we are) so I'm usually not too far off that, but if it's a particularly hot or dry day that might change.
When it's just me, I tend to use the braw app on my phone. With the band, we use the korg tuner.
In terms off how to tell if your sharp or flat? If playing with another stable piper, if you have to blow harder to match them, you're flat. If you have to back the pressure off, you're sharp. Sinking the reed in the chanter will sharpen it, seating it higher will flatten in. We've threaded our chanters so that adjustments for as are as simple as "quarter turn sharp/flat" rather than pulling it out, rehemping and guessing.
If playing by yourself, similar principles apply. If you have to blow harder to get a nice tone, you're likely flat. If it's screaming and screechy, you're sharp.
Individual notes can be managed with tape on the relevant chanter holes - however this can only by used to flatten. If you've got a flat F and High G in particular, it's a case of sharpening the whole chanter and then adjusting the other notes with tape to suit.
There's so much more, but this has already turned into an essay and I've got to get back to work. It's a really valuable topic though, so I'll try to come back later.