Evaluating/testing what your staff "know

I know that the retail industry uses 'mystery shoppers' to check for employee honesty and customer service. You might see if you can create a script for selected employees that are not a part of the help staff and have them call. Though it helps more if you have remote sites that require phone support. At those places you can use a 'plant' who goes through the motions of working with a problem.

Another route would be a form of certification and recertification. Consider adapting standardcertification tests for internal skills 'brushups.'

If you can spare the resources, you can always have 'tech support' contests with prizes for those having X number of correct answers. These contests could deal with FAQs and obscure things thatmost people forget unless the problem has just occurred.