In principle, evaluating an OER for use in a course is just like evaluating a traditional resource. However, it will be like evaluating a new title rather than a new edition of a familiar title, since the publisher may be unfamiliar, or the title hasn't been been refined through numerous editions.
Bear in mind, too, that one of the chief attractions of an OER to your students is that it's free, unless they desire a bound edition.
Questions to ask yourself:
OER textbooks aren't just free textbooks (great as that is).
Instructors and administrators at a community college discuss the advantages of adopting OERs in the classroom. One instructor helped develop material; the other adopted her psychology text off-the-shelf.
A technical college English 101 instructor discusses adapting the handouts and other material he uses into a broader OER plan.