saturation point of electrical iron
Saturation in motors is very much influenced by the width of the air gap and the overall geometry of the design. You can run a saturation test and plot a magnetization curve, but it will be difficult to evaluate the quality of the lamination steel. As the motor becomes more fully saturated, the current increases more rapidly as the voltage increases, but there is not a very dramatic or sudden increase. 

It might help if you could refine your question. The first part of the question is quite general, but the final sentence seems to describe a very specific situation that is focused on motors. If you are primarily interested in a specific motor problem, it might help to describe it in more detail. Why are you concerned about the quality of the iron rather than the overall performance of the motor? Depending on your situation, you may want to have a very small sample metallurgically analyzed.