At least 120 days prior to your car registration deadline, find a RAPIDPASS location, which vary every week all around Northern Virginia on major roadways (the schedule will change as RAPIDPASS systems do not operate during rain, snow, high winds or other adverse weather conditions).
For example, on Jan. 13, there are testing locations in Herndon, Fair Lakes, Fairfax and Falls Church (in addition to Stafford and Loudoun counties). When you pass through a testing location, you may receive a letter in the mail saying your vehicle has qualified. You then have the option to complete your emissions test online (or you can choose to take your vehicle into a traditional testing location).
http://www.fairfaxcounty.gov/news2/attention-drivers-your-emissions-test-just-got-easier/