If you know any reliable garage, bring the car there to have a full inspection.
If you don't have one, the most reliable way is to bring the car to AA car club. They provide car inspection service for second hand car before you buy it. It costs around $1500 but definitely worth it. Can save you much more than you pay for that. http://www.hkaa.com.hk/sc/sec_content.php?subsec_id=57
Buying new car is different. You more or less know about the car and just want to test drive it. In fact there is a very convenient way, call up the dealer and ask them to bring the car to your home for you to test drive. I bought my last few cars this way without walking into the shop.