Here are your options:
1. Listen to the scanner on the Fairfax scanner site at the time the cops are there.
2. Listen to the scanner on the radio reference site at the time the cops are there.
3. Buy your own scanner and listen to it at the time the cops are there.
4. If the cops were there on a reportable incident then they will put it in their weekly summary that you can read here:
http://www.fairfaxcounty.gov/police/crime It normally takes a week before it makes it into the report.
5. You can call the local police district office and ask why the cops were dispatched at that time.