How to determine - ATF or synthetic Pentosin?

[eyvind.spangen@c2i.net (Eyvind Spangen)]

My '87 5k has its reservoir topped of (yeah, it's overfilled) with
some red fluid. No leaks anywhere, but I don't know if it's a person
not knowing the 5k hydraulics who has filled it with ATF, or if
somebody a bit more competent used the synthetic Pentosin. How can I
determine that, and how do I change all the fluid completely? Will
change that whatever-it-is stuff with the orginal VW G002000, as the
Pentosin brand is unavailable in this part of Europe..

ES