Because you shouldn't. Either try and write, and see if it fails, or if need-be ask for an exclusive lock on the file and see if the OS gives it to you. That isn't just permissions either, it actually picks up a lot of potential problems (e.g. device disconnected, lock conflict, et al).
This. Seeing if you can do something before actually doing it falls victim to TOCTOU[0] bugs. Until one actually tries to perform the security-sensitive operation, there's no guaranteeing that you will succeed.
