Red flags I've run into on freelancing websites:
* "I don't like this article, send me another, and another". And I've caught them putting my articles up on various sites after submission.
* Rejecting the article almost as soon as it is submitted. This lets them avoid payment. I now promptly post to Hubpages any article rejected on any freelancing platform, so they can't use it without paying me.
* "Send me some new content as samples" - so they can use it and you now can't use it elsewhere.
* They don't have any way to be reached other than an email address.
* Lack of defined goals for the content, so they keep sending it back for revisions - and intentionally get several rewrites that they can publish as free spun content.
* Their reject rate is sky high.
* When you are asked to send a large batch of articles at once instead of being paid as you go.
* Asking you to plagiarize others' work. I don't put "summarize this WSJ or Economist article and explain what it means to X industry" in that category.