Verifica se existe pasta dentro da pasta c do celular

File dir = new File(Environment.getExternalStorageDirectory() + "/myDirectory");
if(dir.exists() && dir.isDirectory()) {
    Toast.makeText(getApplicationContext(), "Directory Found", Toast.LENGTH_SHORT).show();
} else {
    Toast.makeText(getApplicationContext(), "Directory Not Found", Toast.LENGTH_SHORT).show();
}