Your best bet would be go with a’branded’ dealer, who are usually part of the car companies. For example, Maruti has its "True Value" prorgam, under which is dealers sell used cars. Though this would be around 10 - 15 per cent more expensive than going with an unbranded dealer from the unorganised second hand car market, you will have the peace of mind of getting a good car, that is usually backed up with a service gaurantee as well for about a year, with some free services included.
Alternatively, you can go in for a multi-brand reseller, such as automartindia, or Mahindra’s used car business FirstChoice India. That will give you a wider range to choose from, since company dealers usually restrict themselves to the brand of their parent. Or what you could do is visit different dealers of different car companies and get your choice of car from there.
If you decide to buy from an individual, the general rule of thumb is that cars owned by Army officers, doctors, and lawyers, are the best - mainly because they are less used, and better maintained.
If you have any specific questions, please feel free to email
Cheers!
Dhruv