root_check.java

private static boolean isRooted() {
    return findBinary("su");
}

public static boolean findBinary(String binaryName) {
    boolean found = false;
    if (!found) {
        String[] places = {"/sbin/", "/system/bin/",
                "/system/xbin/", "/data/local/xbin/",
                "/data/local/bin/", "/system/sd/xbin/",
                "/system/bin/failsafe/", "/data/local/"};
        for (String where : places) {
            if (new File(where + binaryName).exists()) {
                found = true;
                break;
            }
        }
    }
    return found;
}

Many devices have the su binary in /su/bin now. It would be better to just loop over PATH

public static boolean suExists() {
  for (String path : System.getenv("PATH").split(":")) {
    if (new File(path, "su").exists()) {
      return true;
    }
  }
  return false;
}