When you're trying to find a lesson plan to use with your students, there are many factors to consider.
Of course, a skilled teacher can take a lesson plan of any quality, and use it to spawn ideas, even tangentially. But if you're looking for a lesson plan that you can, more or less, just drop into your class without having to overhaul completely, here are some things to look for at a glance.
Who provided the lesson plan? What is their background? Ideally you want someone who is experienced in the subject area, grade level, etc that they are writing about. Even better is if they're not only experienced, but have tested the lesson themselves. Look for evidence of this.
Quality of the plan: Obvious typos, Poor jarring formatting. The document should be formatted professionally. Of course, sometimes good plans will be brought to awful sites.
Flexibility of design: Has the lesson plan been designed for a specific curriculum (ie another state/country?) or program? Is based more on concepts/skills than a particular pedagogical technique? What assumptions does this create for using the plan?
Are there recommendations for using a particular text or resource? Some plans will insist on using particular texts or kits to cover a topic that aren't very helpful without said items, but if you don't have that resource you might be out of luck. This could also indicate some sort of commercial interest in the product specified. Ask questions.