keep in mind that you most likely will have the power valve open when you are into the secondaries.
The best way to determine if you need to change them is to run the car in the range where the secondary main jets are going to be active. Make a pass and shut it down and coast to a stop, then remove the spark plugs and see what they read. If you idle down before reading the plugs, it may affect the readings, that's why you have to do a "spurt" and then shut it down with the trans in neutral and stop the car to read the plugs like at a drag strip or deserted road.