Another way of telling is by the signal stream using the GPSinfo software on the setup tab select the WAAS checkbox. In the NMEA sentences look for the line below.
$GPGGA,170834,4124.8963,N,08151.6838,W,2,05,1.5,280.2,M,-34.0,M,,,*75
If you see a 2 in the bold spot you've got a DGPS or WAAS fix if it's 1 then it's ordinary GPS fix.
Further infomation from: http://home.pacific.net.au/~gnb/gps/nmea.html#gpgga