$isfilevalid(file descriptor)

This function returns true if the file descriptor was previously returned by the open function and has not yet been passed to the close function, and false otherwise.

This will tell you whether or not it possible to read or write to a file.

 1   The file descriptor is valid and can be used.
 0   The file descriptor is not valid and cannot be used

This function first appeared in EPIC4-1.1.6.