The 5 steps to taste an espresso

Analyze the espresso cream (it's the first impression).

Because of the 9 bar pressure generated on top of the ground coffee, espresso is the only system for making coffee with cream.  The color of the cream indicates the type of roasting applied by the coffee, the temperature of the water,... The consistency and persistence of the cream is an indication that the extraction is done correctly by a barista and the coffee is fresh from roasting.

Bring the nose closer to the cup to perceive the aroma.

If you want to perceive the aromas better, remove the espresso with a spoon while you smell the vapours that come out under the cream.  Aromas of fruits, nuts, chocolates appear and tell us how the flavor will be.

Savor espresso when it's hot.

Sip the coffee with a sip without and (if you want) with a little sugar to know how most people will perceive this coffee.  Sugar will diminish the bitter and excess acidic taste we sometimes find in badly roasted/prepared coffees.  Cold espresso coffees don't usually taste good (or else, you found a diamond)

Aftertaste" for a long time.

The residual taste that remains in your mouth after taking it can be perceived for a long time.  And if it is of good quality, invite to come back or take another!  This is what we have to achieve as good baristas.

Feedback is important to the barista.

In order to improve the coffees we drink in the cafeterias, it is vital that you say whether you liked it or not... and why not.  An excess of bitterness and acidity, burned, fermented and dry wood flavours... are NOT pleasant, and your barista has to know this... in order to improve his service and keep you a customer.  Help him!