The Sparkfun GPS Shield is very handy but it can be hard to debug code indoors if reception is not good.
I added a 6 pin header in the prototyping area with the end pin on the ground bus. You can plug a USB-TTL serial cable (as used with Boarduino, Pro etc) onto this. Patching the 4th pin along (orange on my cable - TX Data) to the TX pin connects the two outputs together. Note the black dot on the shield reminding me which way to plug the cable in.
You can either (i) switch the GPS power on and get real data or (ii) switch the GPS power off, connect a cable and test indoors with any program that can send NMEA sentences to a COM port at 4800bps. I used MS Flight Sim with the GPSout plug-in but there are many apps that will do this.
Of course using this shield with NewSoftSerial you need to be aware that the example sketch assumes that TX and RX are reversed compared to whet the GPSshield actually implements!
Paul