They don't give much in the way of tell tales unfortunately, other than the car feeling down on power, or sounding very diesely (like a tractor) Which is kind of why you need to drive a few before buying one.

The main prob is that they're very expensive to fix, as if one injector goes, the rest will follow.

If the glow plug light flashes and the car goes into limp mode don't even think about it. The fix could be a simple fuel filter, or it could be something more serious and the car will never be the same again.

Have a good look at the receipts (I genuinely wouldn't consider one without a detailled history file) and be aware that some are money pits. Having said that, there are tens of of thousands of good 'uns out there.

If you do get one I'd strongly suggest checking out talkford.com, as there's a lot of Mk3 knowledge over there.