Dan,
Couple of follow-up comments.
SWR mainly affects your radio's ability to transmit. So, if your main use to to listen to the truckers, etc. a poor SWR will not have an impact. If, however you want to use the radio to talk, a poor SWR will have a fairly noticeable effect on your ability to transmit over any distance. Since CB'ers are in moving vehicles, rapidly changing distances can play a big role.
Another way you may be able to verify if you have a NGP antenna is to look at the antenna cable at the junction box. NGP antennas require a special coax. If the coax is thicker for the first ~ 3-9", it's a NGP cable. (At least that was what the cable on my Cobra looked like).
Steve