Wikipedia:Read the source

Be sure to read the source first, before discussing the credibility and/or application of a given source to an article in WP:AFD. Failure to read the source can lead to unclear arguments and frustration among those involved in the discussion. Never assume a source does or does not say something based on just reading the title or based on a "snap judgement" of the origin of the source (e.g., a certain publishing company or university press).

Further, not reading a source can lead others to believe your response is a lie or that you are not being honest. While your intentions may be real and in good faith, your actions may lead others to believe otherwise.

The solution is simple: read the source first, then discuss your concerns with the source.

Example
A hypothetical activist organization, XYZ Activism may have a reputation for putting out "cherry picked, spin-filled articles and reports over the last 20 years. However, the current XYZ report from 2016 being used by another editor may actually use reliable sources (published university press books and journal articles) and an objective writing style. So you can't dismiss the new XYZ report without reading it, because it might be a solid source.