DirecTV does do credit checks, though the CSR running it will not see the information. It's essentially the same process as if you were to sign up for a cell phone.
Basically, you can give information that will help them find your record (you can, by law, refuse to give consent, but you can also deny the SSI# and give your name and address instead), they put it into the computer, click submit, and then it will tell them immediately if you are approved for certain offers.
Usually most people get approved for the best offers (4 room standard installation free with one $100 rebate on an advance product upgrade; 12 month programming discouts; etc.) and if you don't you can still get service, though you may be required to pay $200 or $300 up front (you would then get it back as $5 off per month for 40 or 60 months respectively).
After you start your service and have established a payment history with the company (6 months of service), your account history is then what is used for offers. If you pay on time in full every month (or rarely have your account go past due), don't ever have payment reversals, and keep a minimum level of programming, you are eligible for most (if not all) programming offers to the general market.
I hope that info helps. If you need anything else clarified feel free to ask =)