You can use any character as long as it is unique or you have enclosed the field data in " during export.
E.g. you could use $char(9), the tab character to separate the fields.
There is no internal function that can be set by a menu function to determine the separator, you enter (and choose) it yourself.
If you deal with a complete library then it is probably advisable to get the fully qualified filename. Otherwise you run into trouble if you have tracks on a compilation and on the original album...
To get the correct data, it must have a unique identifier - that is the rule.