slant valve springs

No, I would not change the springs.

You can do a rudimentary check by measuring spring free length; any that are way off from the others are suspect. But the only right way is to compress to a specific length and measure the spring force with a spring tester. My old manual for solid lifter /6 lists:
78-88 lbs @ 1-11/16"
170-184 lbs @ 1-5/16"