I only talked about tasting last time, and Walt rightly pointed out that you can’t divorce that from smell and other sensations. The reason I didn’t mention those was because I wanted to talk about them separately.
Let’s start with smell. Smell, I think, is one of the most elusive and difficult to discuss traits of a tea. One problem with smell is that they are extremely fickle, and everyone has a different idea of what something smells like. Generally speaking, I find smell to be very unreliable in evaluating teas, especially things that are non-puerh. They are also harder to tell apart — so a cheap tieguanyin may not smell so different from an expensive one, so on, so forth.
There are a few things that you can use smell for though. The first, I think, is storage condition for older teas, and not just puerh. Obviously, if a puerh has been traditionally stored, there’s often a traditional storage smell (which will manifest itself clearly in taste as well and appearance). Also, for newer, drier stored things, the smell can often give you some clue as to how the tea was made and what it’s like. Smelling dry leaves can be deceiving, however, whereas smelling wet leaves or brewed tea can give you a lot more info. I’ve been drinking some randomly purchased ~5 years old puerh recently, and some share a distinct “stale green tea” smell — teas that, I think, will not age well in the long run. Good puerh will have a solid change by 5 or so years, accompanied with a thickness and depth that is lacking in some of these “stale green tea” types. I can’t quite describe how they are like, but I know one when I see one.
Aged oolongs can also be evaluated using smell, in this case partly thanks to how the tea has been stored — has it been roasted? Stored well? Does it smell sour? Aged? New pretending to be old? All those things, with experience, are at least somewhat discernable using smell. I think the same principles can be applied to every tea, to a greater or lesser extent, and smell acts as a confirmation signal — it can help you figure out things, but on its own, can be somewhat misleading.
Now, the other aspects of tasting a tea is more ephemeral. I’m talking about what I normally call “depth,” which really means how a tea feels when drunk. There are two parts to this. One is a physical reaction on a sensory front — how a tea feels in the mouth, and how it feels down the throat. Good teas often will trigger a reaction in the throat area, as well as feeling very full and thick in the mouth itself. It coats the mouth with sensory stimulation that weaker teas do not provide, and this is often the difference between an ok tea and a great tea.
The other is something even more difficult to describe, and which some will call “qi,” meaning energy/substance/stuff in Chinese cosmology. It is difficult to explain what it is, but I think the best way I can describe it is that it is a physical reaction to a tea that goes beyond the mouth, throat, and stomach. For me, it manifests as a sensation that creeps up my back. For others, it’s a different reaction. Great tea will usually be accompanied by this — an obvious sense of qi rushing up. It is something special, and a lot of teas do not have such a thing. This is not to be confused with a caffeine high, however. They are most definitely not the same.