To find it with certainty, you'll need to:
* Consult your owner's manual: This is the best resource for finding a diagram of your specific engine and its components.
* Use a repair manual: A Haynes or Chilton repair manual for your vehicle will have detailed diagrams and instructions.
* Search online for a diagram: Search for "2007 Chevy Silverado [your engine size] thermostat location" to find images and videos.
Keep in mind that accessing the thermostat often requires removing other components, and working on the cooling system requires letting it cool down completely and taking proper safety precautions. If you are not comfortable with automotive repairs, it is best to have a qualified mechanic replace the thermostat.