I know I'm way late to this thread, but Pixie, the dealer or a good mechanic can usually determine whether an aftermarket performance chip has been added to your car. Many times visual inspection of the ECU (for evidence of it being opened) and the chip itself (reading the markings/lettiring on the back of the chip)will give it away, and the computer scan tool can also usually tell if the chip is aftermarket or OEM when the ECU is read by the scan tool.
A chip itself, on a non-turbo car, with no other modifications, only increases performance a small amount. We're talking 10hp max, so while it may be noticeable to a well-calibrated fanny, it won't cause a rip-roaring increase in performance. So instead of running 0-60mph in 7.7 secs, maybe you can do 7.5 secs. I know because I have a chip in my '96 VW Jetta. It doesn't do all that much.