Differences between AU models?

[mickbag]

I'm a newby - my first post here altho lurking for a week or so...

We recently bought an AU3 Fairmont at our place. We're very happy with it (so far)... certainly good value for money. My wife is keen to know (as would I) how you can tell the difference between the different AU models? Are there any obvious external differences between, say a 98AU, a series 2 and a series 3?

Thanks in advance for your knowledge and input...