Yes, they checked license plates and CB call handles.
I really wish they had taken pictures of the writing. I strongly suspect part of it had been erased, but at this point there's no way to tell.
Just for reference, here's the detailed description (from
http://www.pamissing.com/bethdoe.html):
There were letters and numbers written in ink (color of ink unknown) on the palm of the left hand of the victim. While there have been different interpretations of what was actually written on the hand, almost definitely was WSR. Next to that was either a 4 or a 5. Below and to the right was either a 4 or 7. Police checked license plates and CB call signs but were unable to determine the significance of the notations. The ink would have probably lasted 8-12 hours on living flesh.