The GoMog article is a good general primer.
Unless you, or a previous owner changed to silicon brake fluid, chance are what was in there when the car was new was DOT4. Possibly changed to DOT5.1 at a later service.
But if the car hasn't had a brake fluid change for 3 or more years, the whole lot should be renewed, and I'd be using DOT5.1 as it has a higher boiling point (I have boiled brake fluid...). I'm aware the GoMog article says a 2 year life, but in my opinion, with a car that lives in a garage and does not wade, 3 years is ok.
As the GoMog article says DOT 5 is silicon and must not be mixed with anything else.
