How do I select for combining ability?
The ONLY way to select for combining ability is to test for it.
Even though there is a positive relation between overall vigour of an inbred and the yield of its crosses, the combining ability is more important. The breeding value of a certain hybrid is determined by studying it's progeny.
Making an inbred-to-varietal cross (top cross) is one way. Cross the various lines to a stable variety (Skunk no.1, Northern Lights, etc.) and the progeny that produces the best crosses is selected. Repeating the test in different locations will eliminate any possible influences the environment might produce, and repeating the test with different testers would ensure that the results were accurate.