Just a hint - if the word ends with Ы - it is most likely plural. Examples will be Часы, Кактусы, Фикусы, Примусы, etc. However, and I am sure you know that, there is a word ЧАС which means an HOUR.

Yes, the word "часы" is always plural. "I bought a watch." - Я купил часы. (одни) "I bought two watches." - Я купил двое часов. We use the indefinite article "a" or "one" in English to show that the noun is used in singular. We don't use any articles in Russian in singular as none exist in Russian. We use a numeral "двое, трое и так далее, or a pronoun (несколько, много) to show that the noun is used in plural.

It must sound funny to a learner that we have a plural form of the word "один" - одни часы, одни очки, одни брюки :)

Это зависит от того, что Вы имеете ввиду. Если механизм, то в единственном числе, если время по СИ, то - во множественном!