All of the NASA aircraft can be tracked here NASA::Airborne Science Program::Aircraft Tracker then you'd have to determine LA center or Oakland CTR or SLC Center. Based on what you see and where they are. Better to find the freqs they use for missions. 235.4 for starters. You'd also want the Joshua ATC UHF freqs for the Edwards area and ranges. And Palmdale Plant 42 as well.
Try:
296.700 Oakland Center
369.900 Los Angeles Center
364.800 Salt Lake Center
285.450 Seattle Center
The above frequencies are used for flight operations above 60,000 feet (FL 600), which is the altitude U2s operate at. Due to the fact that they are at extremely high altitudes, transmissions from these aircraft can often be heard from hundreds of miles away.