An experienced salesperson should recognize that you aren't buying today and won't pressure or pester you. If they do, remember their job is to make a sale, so don't fault them for persistence. Assure them that you're seriously considering their vehicle for purchase, and if you choose it, they'll be the first to know. But don't let yourself get roped into a purchase during the beginning of your search. Politely make it clear that you are there for a test-drive.
Most dealers insist that a customer contact sheet be filled out prior to a test-drive.
Allow them to fill it out, but don't sign it or furnish any credit information, as lots of credit checks on your credit history make you look bad. The only document the dealer legally requires before a road test is your driver's license, and sometimes proof of insurance. Don't discuss financing or let the salesperson know whether you're trading in a car. If, after the test drive, you feel ready to wheel and deal, don't. You won't know what you've missed unless you test- drive some more cars.