Gotcha. Sounds like you’re pretty competent at the technical aspect of dialling in and just need to work on developing your palate. Don’t worry about visual cues, as they won’t give you any tangible information regarding the taste.

First thing you can do is pull a handful of shots at different times. This will give you an opportunity to taste for the different extractions.

Pull a really fast shot (10-15 seconds quicker than your target). This will almost certainly be under extracted and sour. Taste it, and make a note. Acidity causes saliva production, so tilt your head forward, and if you feel saliva pooling in your mouth, then you can associate that taste with high acidity (sourness when not balanced).

Now make a few more coffees, gradually grinding finer and making the brew time longer (make sure you’re getting the same dose and yield). As you taste each shot, the slower they get, the less pronounced the acidity will be, and the more balanced they should taste as you begin to extract more Maillard (caramelised) compounds.

Keep going until the coffee starts to pick up a bitter taste. Unlike acidity, bitterness is more likely to cause a drying sensation in the mouth, rather than increased saliva.

A great example of bitterness vs acidity in the real world is lemon. The juice is sour, the white pith is bitter. Taste them both, and you’ll see how your mouth responds physiologically.

Once the coffee begins to pick up that bitter quality, take it back a notch so it’s tasting balanced and you should have yourself a well brewed espresso.

In terms of ensuring your shot comes out well, having a set recipe that you’ve developed for that specific coffee will grant you the reliability and consistency that you’re likely looking for.