Do active researchers in mathematics read a paper in detail?
The answer, of course, is that it varies with each reader and with the specific needs of each reader. For many people, a skim is sufficient for most papers. The question is "does this seem reasonable" and if so, there may be little need to go into the details. 

This is especially true about proofs. If an overview of the proof suggests that the techniques are standard for the field, and the results don't seem to clash with what the reader knows, then it is unlikely that the reader will spend the time on every detail. 

The exceptions, of course, are many. Students want to learn new proof techniques that they are relatively unfamiliar with. Surprising results require a close look. Long standing problems, when finally proven, also require a close look. 

And, in many cases, the way something is proven is more important than the result itself. If a skim of a proof suggests that there is something new here, then experienced mathematicians will want to examine it in detail as well as look for gaps and errors. 

But if you are a student, or otherwise feel the need for an answer to this question, then I'd suggest that you err on the side of completeness. Keep your skeptic hat on until you are satisfied.