Is "brings all the boys to the yard" an idiom? How to find out if a phrase is an idiom or not

Google Books Ngram Viewer is a useful tool for this sort of problem. Using "boys to the yard" as a search term, we get:

As you can see, the number of occurences is miniscule, so it is fairly safe to assume that brings all the boys to the yard is not idiomatic.

Unfortunately, the maximum number of words allowed is five, so we cannot search for the entire phrase.

Despite the literal nature of the music video, it's not literal. In fact, trying to rationalize a literal explanation is a bit absurd, as you can see here.

In case you're still not convinced it's not literal, read Kelis's response in a recent interview (emphasis mine):

What would go in a Kelis milkshake? Apart from milk, obviously …

I don’t think I’ve ever made a milkshake. I made a smoothie the other day that was really good – protein powder, spinach, orange, and crushed ice with a little bit of honey and coconut water.

However, I wouldn't describe it an idiom. I like how Wikipedia describes it:

According to Kelis, "milkshake" in the song is used as a metaphor for "something that makes women special." The song is noted for its euphemistic chorus and low beat R&B sound.

It doesn't have a concrete meaning, which is typical for many songs.

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As for idiom dictionaries, yes, they exist. Wikipedia has a page about it, and you can find a lot by searching.

Wiktionary has a large list of idioms too.