Learn Spanish (Latin America): Los pasamanos se limpian cada día.

In the reflexive passive (pasiva refleja), the thing being acted upon is actually the grammatical subject of the sentence. Since Los pasamanos (The handrails) is plural, the verb must also be plural (se limpian) to agree with it.

How do we know this is the reflexive passive and not the impersonal "se" (like "one cleans the handrails")?

The key is subject-verb agreement. In impersonal sentences, the verb is always strictly singular. Because the verb here is plural (se limpian) to match the plural noun (los pasamanos), it is the reflexive passive.

Why doesn't the word pasamanos have a different plural form?

Pasamanos is a compound noun made up of a verb (pasa) and a plural noun (manos). Compound nouns that already end in an "-s" do not change their form in the plural. You just change the article to show the number: el pasamanos (the handrail) becomes los pasamanos (the handrails).

Could I say Los pasamanos son limpiados cada día instead?

Yes, that is the true passive voice using the verb ser. However, in Spanish, the reflexive passive (se limpian) is much more natural and common for everyday actions where the person doing the cleaning isn't specified. The true passive often sounds very formal or journalistic.