A lot of people use Google's n-gram to see if something is idiomatic, but for a lot of non-native English speakers even that doesn't really help, so what would you suggest non-native speakers to do so that everything they write sounds 100% idiomatic?
Find beta readers who are native speakers of English and give them red pens and completed manuscripts (which you've already run though Google and done the best edit you can on them) or the online equivalent.
When you've completed the next several rounds of editing, hire an excellent proofreader who is a native speaker.
The only other answer is "spend years getting more fluent in English." While there's truth to that, it's not reasonable. And if you didn't grow up speaking English every day, there are subtle things you will miss. So getting outside help is the way to go.
Frankly, even native speakers need good proofreaders. Even native speakers who are professional editors. You shouldn't be the only editor of your own work.
"Her English is too good, he said. Which clearly indicates that she is foreign." -- professor higgins (my fair lady)
Being overly proper is often a non-native speaker indicator.
But this is not what you are asking...
Google n-gram is NOT a source of idiomatic expressions. n-gram is just a collection of frequencies of words appearing in a sequence (of n) over a large corpus of text.
It really doesn't tell you if an expression is legit or not.
Idioms are part of language, though how a work could be 100% idiomatic seems bizarre.
Oscar Wilde once said we are separated by a common language - that is true. The differences between English spoken in different locations can be a source of confusion or amusement.
Do you intend to write the piece with a strong regional flavour?
Idioms are often a difficult thing to use correctly for non native speakers. Idioms are learned locally and vary from place to place.
In some parts of the US, if you buy a Coke, you bought a soft drink, in others a pop, or a soda, or a soda pop. Same bottle of cola but referred to in multiple ways dependant on location.
Idioms are interesting in that their use will flag a speaker as being from a particular location even if their accent is similar to the local one.
The problem with idioms is that non native speakers often misuse them. They can be confused by the multiple meanings of a word and be unaware that in the region they are visiting, that word means something else.
If, for example, someone who learned English in a formal setting and knows that pop is a sound, or a brief light hit might be puzzled when they hear someone wanting a pop.
Find a proof reader who was born and raised in the area you are writing about and let them teach you the proper use of the local idioms.