If you are just minimizing caffeine and still would like to order your beverages with decaf espresso, there is no way to tell the difference taste-wise.  Some coffee enthusiasts will boast their ability to taste the difference, but coffee sommeliers worldwide would beg to differ. You should always ask if you are unsure whether your barista made your drink decaf. Especially if you are pregnant or have health conditions that are worsened by excessive caffeine.