Make sure that the salesperson is backed by a nationally recognized company, and check out the references. Be aware that the government mandates a three-day rescission period, giving you an opportunity to change you mind. If you choose to purchase a system, the company you are dealing with should not call you, send you literature, or begin the installation before the fourth day. As for the quality of the installation, make sure that the system is installed according to code. You don't want to be stuck having to bring the system up to code, should you choose to sell your home.
Many companies make claims regarding the quality of their own brand.
One way you can verify these claims is to check to see if they are certified. They must meet the requirements of strict public health performance standards developed by NSF International. For a system to become NSF Certified, it must meet five basic requirements. The first requirement assures that contaminant reductions claims are true. The second assures that the system does not add anything harmful to the water.