Joel says there're on you two relevant parameters to measure: "smart" and "gets things done".
Together with DP we came up with easier, down-to-earth and more engineering-oriented quantities directly measurable in a human conversation:
Bandwidth is how fast one can think to produce output. This might not always be important, but usually it is, especially when estimating price-performance ratio to make the hiring decision.
SNR - Signal to noise ratio - is the ratio of meaningful, smart, relevant and correct things s/he says to the total audio output. It's usually up to the interviewee to balance the trade-off between SNR and Bandwidth, but you should be able to factorize by it to deduce the total mental capacity.
All other skills you should be able to teach the employee yourself. If you cannot teach, don't hire anybody. Relevant background = oxymoron, if you are going to develop only new things. Remember - specialization is for insects.