Learn Italian: La bevanda sa di mandorla amara. - The beverage tastes like bitter almond.

When sapere is followed by the preposition di, it acts as a sensory verb meaning 'to taste of' or 'to smell of'. Since the subject here is La bevanda (the beverage), sa di translates naturally to 'tastes like'.

If sa di can mean both 'tastes like' and 'smells like', how do we know which one it is in this sentence?

It relies entirely on context! Because the subject is a beverage, tasting is the primary sense being used. If the subject were something like Il profumo (the perfume) or L'aria (the air), sa di mandorla amara would mean 'smells like bitter almond'.

Why is it mandorla amara (singular) instead of plural, when in English we might say 'tastes like bitter almonds'?

In Italian, when describing a flavor or essence using sapere di, it is very common to use the singular form of the noun. You are referring to the general essence or flavor profile of a 'bitter almond', rather than literal, individual almonds.

Could I use the verbs assaggiare or gustare here instead of sapere?

No. Assaggiare means 'to taste' in the sense of sampling a food to see if it's good, and gustare means to savor or enjoy a flavor. When describing the inherent flavor that an object possesses or gives off, you must use sapere di.