It's "overalls are". It's incorrect to say "overalls is" because this term is always used in a plural form.
It's "overalls are". It's incorrect to say "overalls is" because this term is always used in the plural.
Your overalls are dirty.
Your overalls is dirty.
Conversely, the expression "a pair of overalls" takes a singular verb.
There is a pair of overalls in my closet.
There are a pair of overalls in my closet.
Follow the same strategy with other verbs, such as have, fit, look, etc.
Your denim overalls look great when you wear them with t-shirts. (not "looks great")
Keep also in mind that we use a plural determiner or pronoun (these, those, they, them, etc.) with plural nouns.
These overalls fit perfectly. Do you like them?
A partial list of clothing-related terms that only have a plural form, and consequently take "are" (a plural form of a verb):
