You would need a digital scanner. Among those are the Uniden BCD396XT, BCD436HP, BCD536HP, BCD996P2, and BCD996XT; the Radio Shack PRO-668, PRO-18, and PRO-651, and PRO-652; the Whistler WS1095, WS1065, WS1080, and WS 1040; and the GRE PSR-800, PSR-500, and PSR-600.
One thing to keep in mind is that even with a digital scanner, Fairfax County's digital transmissions can be somewhat garbled depending on your location. This is due to a situation called simulcast, or LSM. A directional antenna pointed at a certain radio tower may help.
I would recommend doing a search in the Virginia forum for "Fairfax P25" or simply "Fairfax" to learn what locals have said about the system. Fairfax is outside of my receiving range.