You'll have to disassemble the heads/valves to get the springs out. Once they're out, you put them on a spring tester. It basically compresses the spring and tells you what type of pressure it is resisting at certain heights. You can always shim them to 'get' the right pressure, but I don't recommend it.
FWIW, I just finished building up my 331 and tested all the springs while they were out. Ended up that 7 out of 16 springs were out of tolerance, so I replaced all of them.
Hope this helps.
E
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1991 5.0 LX Coupe - 40,750 miles
331 cu. in. / Tremec 3550 / BFG Drag Radials
12.22 @ 114.31 mph - w/1.89 60'
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