How do you determine conifer genus and species?

If you know something about botany, use a taxonomic key. If not, use a field guide and look at the pictures. The Golden Guides are surprisingly good. A step up would be the Peterson series.

I agree with anonymous that the "Picture This" app is a wonderful way to accurately identify any plant species.  They also make identification Key's, that walk you through a series of plant attributes and lead you to the correct Genus and species.  If you're in the south, the "Guide to Southern Trees", by Ellwood S Harrar and J George Harrar is what we used in College Dendrology courses.  I'm sure they now would have online Tree Key's available.  However, for simplicity, you can't beat the "Picture This" app.

By taking a picture of it with my phone using the PictureThis app.