To find it definitively, you should:
* Consult your owner's manual: This is the best resource as it will show a diagram of your specific truck's components.
* Look for a large, rectangular module with numerous connectors: The PCM is usually a relatively large electronic control unit.
* Search online for a diagram specific to your year, make, model and engine: A quick search like "2006 Chevy Silverado PCM location" (replacing Silverado with your specific model) will likely yield images and forum posts that pinpoint its location.
Warning: Before attempting to locate or access the PCM, disconnect the negative terminal of your battery to prevent accidental electrical shorts.