See Figures 1 and 2
Valve spring squareness, length and tension should be checked while the valve train is disassembled. Place each valve spring on a flat surface next to a steel square. Measure the length of the spring, and rotate it against the edge of the square to measure distortion. If spring length varies (by comparison) by more than 1.6mm or if distortion exceeds 1.6mm, replace the spring.
Spring tension must be checked on a spring tester. Springs used on most Toyota engines should be within one pound of each other when tested at their specified installed heights.