My method runs thus:
Scan the spines in the desired sections (in my case, Science Fiction & Fantasy first, then Horror, then Fiction, then Literature and Literary Criticism) for recommended authors and inspiring titles; pick the above off the shelf to evaluate the cover, inspect the blurb and read the "Praise for" bits at the beginning. Lastly, open somewhere down the middle and get a feel for the style.
If all of the above go well, take to the till !