Not a linguist but in my experience you simply cannot tell in advance and have to infer from context clues. The rule is: If they mean it as a plural, it is plural. If they mean it as a singular, it is singular. English has a lot of irregular words due to all its linguistic influences.
Words that stay the same when referring to the singular and the plural (such as “squid,” “sheep,” “fish,” and “species”) are a subset of [irregular nouns](https://proofed.com/writing-tips/irregular-plurals-spelling-tricks/#:~:text=Some%20irregular%20plurals%20actually%20stay,”%20(plural)%20are%20correct) but irregular nouns is a big category that includes more than just what you’re asking about. The good news is that [uncountable nouns](https://proofed.com/writing-tips/countable-uncountable-nouns/) (like “water” or “sand”) always stay the same regardless of the amount being described so there is a little bit of structure for you at the very least.
Latest Answers