For us, it's a combination of several factors. Partly it's the reputation of the author: if we've made recipes by this person (or book, or magazine) before, what other people have to say about them and their recipes, what we know of their style and audience. We also read the recipe to see if have trouble following it and to figure out if there's a wide or narrow margin for error while cooking.
But it also comes down to instinct more often than not. We've been cooking long enough that we can look at a recipe and size it up pretty quickly. We also know our own skill level and can tell whether or not we'll be able to improvise if we hit a snag.
How do you evaluate recipes? What makes you trust one over another?