You should change your batteries when the no longer hold about 70% of there rated capacity
It depends on how well you take care of them.

Checking under load is accomplished with a tool called an ESV (expanded scale volmeter) it test the capacity while at the same time drawing current (load) this is what is meant by checking under load. my tool test 4.8v 600mah nicads with a simulated 100mah load

Also go over to the battery forum and do some reading