How can I develop an "ear" for piping?

Contact a vendor (check out the links page) and get recordings of bands and individuals who have recently won or played at "World Class" levels. After listening to recordings of excellent players, you will have a much better idea of the range of moods and expression that a well played bagpipe can achieve.  From that point on, when you listen to pipers, you will be readily be able to hear the differences between a good piper and a "marginal" player. Some of them are:

• A good player will have absolutely steady, rock solid drones with no wavering in the pitch. The cause of wavering drone pitch can also be seen visually as any waving about of the drones as the pressure in the bag varies while playing.
• Tuning will be excellent. The bagpipe is a loud instrument, but the bagpipe should have a rich, resonant, pleasant sound. If it doesn't sound "good", it probably isn't well tuned. It takes a few years of practice and a good ear to learn to tune a set of pipes.
• The tempo of tunes will be steady and not wander around. You should want to tap your foot.
• The piper will not "lose" notes, especially in the difficult passages. These moments when the chanter stops making sound are called chokes. The bagpipe should "speak" continuously.
• The playing should be clean, clear and crisp.
• The sound will be warm, rich and enveloping.
• The bagpipe should start up with drones coming in well tuned, then the chanter will sound - also well tuned. After the tune is played, the bagpipe should be silenced "crisply" and never allowed to deflate noisily.