Car wax is pretty incredible stuff. Not only does it make your paint look great, but it also prolongs its life. However not everyone knows when exactly their car needs waxed and how often. Once a week? Once a month? Once a year? First, let's toss out the myth you can get away with one of those "Once a Year" car waxes. All waxes will start to degrade after 30 - 45 days on the surface of the vehicle no matter how good they are. In fact, Consumer Reports put out a test several years back proving this point. However, that's not to say that you should skip waxing your car entirely.
The tried and true method to tell if your vehicle needs a coat of wax is the water drop test. The tighter and taller the bead, the more wax is on the surface. The flatter the bead, the less wax there is. Quality car waxes will make the car's paint "hydrophobic" which basically means it repels-water. The more hydrophobic the surface, the tighter beads you'll get.
The photos below show how car panels look with and without wax. On the second panel we applied Eco Touch Spray Car Wax and buffed dry. The results speak for themselves...
Another method which is slightly less scientific is to run your fingers over the paint. If it feel "grippy" and not slick you may be due for another coat of wax. Just remember that this test would best be done when your car is cleaned. If there is a layer of dirt on top of the wax, you won't be 100% sure of the protection. James Dudra on Google+