startsWith

Determines whether this file belongs to the same root as other and starts with all components of other in the same order. So if other has N components, first N components of this must be the same as in other.

fun File.startsWith(other: File): Boolean(source)

import java.io.File

fun main() {
    val root = File("/home/user/projects")
    val fileInside = File("/home/user/projects/kotlin/Example.kt")
    val fileOutside = File("/home/user/other/Example.kt")

    println(fileInside.startsWith(root))   // true
    println(fileOutside.startsWith(root))  // false
}

Source