How do I check for system power to the thermostat? Presently there is no voltage showing in my thermostat. Power to my system is present at the power switch, but beyond that I have no idea what to check. The unit is a Coleman Evcon DGAT056BDF.

Many thermostats bring in "R" usually the red wire and switch that to "W" which is heat, "Y" to the AC contactor outside, and to "G" the fan relay. Some will bring in the common wire on digital thermostats, but not on yours, digital thermostats are set up where you can test the voltage between "R" and "C" to see if you have voltage because they need a complete circuit to work. Only a few do not need a common wire. To test it start at the output of the transformer and you will get from 24 to 30 volts. One wire is "R" and the other is common. Always start there. You can then check each component. It's so easy to do you should not have any problems. Here are some diagrams: https://dl.dropboxusercontent.com/u/8010163/AC%20Wiring%20Diagrams.zip
https://dl.dropboxusercontent.com/u/8010163/How%20to%20use%20Test%20Equipment%20Service%20Manu al.pdf
If you have any problems go to the library and check out a book on air conditioning and heating electrical systems. It's all very simple.