Yeh, you can tell if it needs doing, it becomes less and less clear and more brown.
The test they do is to see how much it has been watered down by condensation (debatable).
Ford recommends every two years/25,000 if you've done that or you're hard on your brakes then go for it.
If it still looks quite clean then leave it another year.
I did mine at 3 years on my Focus and it was still quite clean TBH.
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2011 Focus 3-door Zetec 1.6 TDCi.
Previous <img src="/images/graemlins/stemo.gif" alt="" />s - 2006 Panther Black & 2007 Red. Gone and quite forgotten <img src="/images/graemlins/lol.gif" alt="" />