How to detect what fluid is in my transfer box?

I'm about to top up my transfer box and axles but I have no service history and no idea what's currently in my transfer box. If I had a straightforward case, I'd just put some EP90 in, however, knowing that some weirdos do use ATF in their transfer boxes, I'm wondering if there's a quick test I can do?

It's a manual 300Tdi, 1996.

I expect the stuff will have blackened by now, so I couldn't judge by the colour. Got a leak in one of my axles and the thing that's coming out is definitely black.

If I end up putting EP90 over some old ATF, will it turn into cottage cheese?

Why topping up? Well, I will flush it later, got an urgent job involving a lot of uphill action and I want to make sure all fluids are to the mark.

Check you can get the filler nut off, then empty it and put ep90 in. Takes 2 mins longer than just topping up.

As you have no history it would be a good idea to drain the diffs also - you could be 'unpleasantly surprised' at what comes out.
And have you drained and refilled main gearbox - when I got my D2 a few years ago it had a good bit of history but I still drained and refilled gbox/tbox and both diffs .
What came out of main box was just like water - absolutely no good to box .
And as storm says make sure you can open filler plug(s) before you drain anything .

dooroy, ouch, sounds bad! The car was serviced a year ago and the gearbox, transfer box and axles were drained and filled... with something by a local garage. I have since taken over the maintenance and repairs but I have no documentation on what was actually put in the car.

storm99, I only got 1 litre of EP90 left... but yeah, what you're saying makes sense. I'll get more

You need to buy more..the tb is 1.6 or 1.7 i believe.

storm99, spot on, it's 1.7 for each axle plus 2.3 for the box. Another trip to my EuroCarParts (no, not affiliated ) the last place where I can get the not-made-in-China stuff. British car - British oil... fair deal!