Mine's got sensors, and the valve stems look 100% completely normal - they're just smooth rubber with no nuts.
Mine's a pre-facelift 64-reg. I believe that "proper" TPMS (with transmitter valves) is now a legal requirement within the EU. I don't think the ABS monitoring systems are sufficient any more, but I'm not sure about it - I can't find any clear rules from a quick google, but I'm sure the info's out there somewhere.
If you have steel wheels then, if you compare the spare and one of the others you'll see that even though they're all black rubber, the valve on the spare is a slightly different shape to the rest. This is because it is a standard one, while the rest are sensors. If you have alloys then they may be a different valve anyway, sensors or not.
The best way of telling is to put the spare on and go for a drive. You'll get a warning about a missing sensor, as they only activate when turning, so even if the fourth one is mounted as the spare it still won't be transmitting. It might take 10 or 20 miles to tell you about it, so go for a decent length journey.
Don't let a tyre down to test it! It won't tell you about it while parked anyway, and you don't want to go for a drive with a flat just to try it out.
This is what they look like...
Don't rely on the cosmetic appearance of the stem though - there are several different makes that look slightly different. But they do look exactly like standard valves when fitted, so there's no easy way to tell.