Without going over to the dealer to confirm, there may be a slight difference in the exhaust tips. Also, there are generally different wheels available, although you could option on the 530 wheels to the 525.
Other differences I can think of from years past would be in the side skirts below the doors. Older 318's had gray and 325's were color matched.\
Also, being that the 530 is a little sportier, it may sit slightly lower to the ground. It would be noticable in the 'wheel gap' that area between the fender and the top of the wheel.
Please note that the above are just guesses as to what would be different. I'm pretty sure I'm not allowed to go look at brand new BMW's, my dealer barely let's me on the grounds to buy parts. :P
2002 BMW 530i
1997 Ford Mustang GT
1995 Honda VFR 750 (For Sale)
1964.5 Ford Mustang