Enter the last 7 digits of the VIN at the following website to decode the options: http://www.realoem.com/bmw/select.do
It's going to depend on how well it was maintained and even then it's a crapshoot. I had a 2001 X5 4.4i with 139K miles on it that was covered under the extented maintenance warranty through the 1st 100K miles and after that I still had all maintenance performed at BMW. After all that, it still had a cracked engine block at 139K miles.
I still have it actually (since the tires are worth more than BMW would offer me for a "trade in"). I bought an '05 4.4i w/ 97K miles for $16 last year. Once the tires on that one wear out I'll put the tires from the '01 on it and then sell the '01 as a parts car (execellent interior and exterior other than the engine problem).
[Dead] 2001 X5 4.4i w/ 139K miles (probable cracked engine block)
2005 X5 4.4i w/ 97K miles