You can watch for changes with this
on *:start:.timercheckfile 0 60 checkfile
alias checkfile {
var %mtime = $file(file.txt).mtime
if (%mtime > %file.mtime) || (!%file.mtime) {
%file.mtime = %mtime
;file has changed do what you want
}
}/
For checking if you can write to the file, I would suggest looking into /fopen and $ferr. I don't think there's a built in alias to give that information.