checking if a file is locked before trying to open it for write?

I've been reading my copy of the manual regarding flock() and it appears that a script will have to try to open the file before finding out if it is locked. So, if another script has a lock on the file and the current script does is_writable on the file, will it return false (and trying to open the file will generate a permission denied error ?). This assumes a multi user situation, like two or more admin users are logged into one cms system and are trying to editing the same file at the same time. Thanks JK