isRooted

Determines whether this file has a root or it represents a relative path.

val File.isRooted: Boolean(source)

import java.io.File

fun main() {
    val absolutePath = File("/usr/local/bin")
    val relativePath = File("src/main/kotlin")

    println("${absolutePath.path} is rooted? ${absolutePath.isRooted}")   // true
    println("${relativePath.path} is rooted? ${relativePath.isRooted}")   // false
}

Source