Jan,
DOT 5 is silicone based and not compatible with any other brake fluid. There is also DOT 5.1 which IS compatible with DOT 3 & DOT 4 brake fluids.
I know of no way to tell them apart.
If you cannot contact the previous owner, or the shop that maintained the vehicle/installed the brakes, the next safest thing to do will be to have the system flushed thoroughly with a power bleeder and refilled with DOT 5.1.
your friend on the DTBB
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