Try taking a PR
final test and compare that score to the other two.
Any commercial company that guarantees a score improvement has a vested interest in showing an improvement from a pretest taken before the class to a post-test taken afterward. The way to make that more likely is to give a hard diagnostic pretest and an easier post-test. One gets a low start score and a high final score. Another way to game the results is to ask more questions that are solvable through the application of that company's test prep strategies, perhaps more than are actually on a real SAT.
Of course, I'm not accusing PR
or any other company of actually doing such a self-serving thing as rigging their diagnostic tests to make themselves look better than they actually are; I'm just saying that that is one way it could be done and that taking a pretest and a post-test without actually taking the class is a way to gather evidence to find out.