For me, if I don't already know the series from an anime or already have prior volumes in a series, I tend to start by reading the title on the spines and looking at the spine art. If any of that catches my eye, then I'll pull it out and look at the cover to judge the art and see the initial cover art to determine if it's too "adult" (basically hentai or way too fanservicey) and if it looks cool. If I like the cover, I'll then read the back and flip through the pages to, again, ensure that it's not too adult. If it's something that I am in to, I'll buy it. Usually, I tend to look for series' that have at least 2 volumes available right then and there so I can grasp a bit more of the story, but I will get just volume 1 if I like it enough.
If they don't have the first volume, or it's so close to being something I might have gotten but for some stupid reason I just don't want to buy it, I'll take a photo of it and see if I can get it elsewhere later.