Is it idiomatic kotlin?

val check = true

when(check) {
    true -> println("it's true")
    false -> println("it's false")
}

For a boolean,

if

is the more readable choice.

When

is better for completeness check on enum/sealed class, or those cases where your

if

has at least 3 branches.

That was my assumptions, but is it common sence or written somewhere?

I don't know if it's been written down anywhere but it's implied. One advantage of the language is being concise, using the when is not as concise as say...

if (check) println("it's true") else println("it's false")

Kotlin's coding conventions mention it: https://kotlinlang.org/docs/reference/coding-conventions.html#if-versus-when