You can watch for changes with this

Code:
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.