Exactly like Crawdaddy posted.
Get down to the nearest Electronics store or if you have a local CB shop that would be better and get an SWR meter.
Without permanently mounting the antenna, use vice grips or a clamp to get it as close as you can to where you want it then start checking the numbers on the SWR meter. You may need to move the antenna forwards and backwards some on the rails to find the best spot for reception.
Mounting a second dummy whip shouldnt affect performance so you wont need it in place.
Also remember to tune the antenna itself, most steel whips can be taken off at the bottom and trimmed down for performance, usually only takes less than an inch or so, sometimes just adjusting it in the holder can be enough.
The SWR meter can be left in line and mounted in the cab if you desire, but normally once everythings adjusted you wont need it anymore except to verify performance once in a while. Most SWR meters come with decent instructions on how to use them, if not do a google search and theres plenty of good info out there on use and adjustment.
Years ago I made the mistake of trying to run dual 102" whips, I had great reception and was picking up radios close to 40 miles away in flat areas and on top of hills, get into the hills or get the radios directly in front or behind me and I couldnt get anything over a mile or two, also no one could read my output it was all garbled and static, the whips were cancelling each other out and causing me to fill the radio waves with trash.
Next project a linear amp, does anyone make them anymore?