Who is responsible for the content? Always check out "About Us."
Are biases clearly stated? Nonpartisan = neutral, free from party affiliation and bias. Consider doing a search on the organization to see what others say about it. Nonprofit does not necessarily mean nonpartisan!
Is there a hidden agenda?
Is it really research? or commentary? Is it reporting? or editorial? Are there sources/evidence for claims made?
What kind of domain is it?