Use is when the subject is one thing: The bus is late. Use are when it names more than one, or is you: The keys are missing. You are right. Some singular nouns end in s, so check the meaning: Mathematics is hard.
Practice: The dogs ___ in the yard (are), My brother ___ at school (is), The list ___ long (is).
When the subject is easy to spot
If you're describing two people doing something together, and makes the subject plural, so are fits: She and her friends are at the fair. You'd only reach for is if the sentence really names one person or thing.
If you're listing two possible culprits and only one can be true, keep it singular: The book or the pen is in the drawer. The or tells your reader one item applies, so the singular verb matches your meaning.
If your sentence opens with a word like each, either, neither, or everyone, you're talking about members one at a time: Each of these hot dogs is juicy. The plural noun right before the verb doesn't change that.
If a phrase follows your subject, check which noun the verb actually agrees with: The list of items is on the desk. Writers often grab the nearer word, items, and write are, but the true subject is list.
Contrast this with an inverted question, where the subject comes after the verb: Where are the keys? The plural keys still governs, so finding the true subject first works the same way in both word orders.
When the wording tests your ear
When the nouns on either side of or or nor don't match in number, the verb follows the closer one: Neither the players nor the coach is ready. It works, but it can sound awkward, so rewording often reads better.
Flip the order and the verb flips too: Neither the coach nor the players are ready. Same meaning, different agreement, because players now sits closest to the verb.
If you're writing for an American audience, group nouns like team or family usually take is: The team is winning. You're treating the group as one unit.
British writers often accept are with the same nouns, so The team are winning isn't an error there. Match whichever variety your reader expects.
One nuance cuts across regions: if you mean the members as individuals, are can fit even in American usage. The family are arguing among themselves again pictures separate people, not one unit.
Your wording tells the reader whether you see one unit or many individuals. Keep that intention consistent within a sentence, because The team is arguing among themselves pairs a singular view with a plural clue and reads oddly.