This is not an indictment on test plots. Many people in companies and universities go out of their way to conduct them fairly and gather accurate results. However, anyone who has worked with plots or who goes poring over test plot data knows they need to keep a few things in mind.
For example, some companies are noted for taller hybrids. A seedsman's nightmare is to have his hybrid planted next to a hybrid from a company with tall hybrids in the same plot. In some cases the difference in height may be one foot or more. It's enough that the taller hybrid can shade the rows on either side of it. It may not seem like a big deal, but if you're in four or six-row plots, shading on even one row which reduces light intake can give an edge to the taller hybrids, all other genetics being equal.
It's harder to tell, but you would like to know where your hybrid fell in relation to field traffic. If it was planted behind the tractor, with more of the area covered by tracks, it might put it at a disadvantage.
You also want to know how uniform the field is. If one area of the field drowns out in normal years, not 2012, and other areas don't, then that field is not a good candidate for a test plot. Or if one is there, you definitely need to make adjustments for the hybrids that wound up on the wetter parts of the field.
The best advice is to look for plots that are actually replicated at least once. This means the entire plot is repeated a second time. Using a checker hybrid may help, where you plant the same hybrid either as every other entry or every third entry, but it's still not as effective as replicating the entire plot. If you use a checker hybrid, the idea is to pick up differences in soil types, and adjust the hybrids next to the checker based on whether the checker performed the same or better or poorer than the field average for the checker.