This is what ‘lancers do, troll around for assignments.  But how to winnow out all the crap from the legitimate opportunities? The former outnumbers the latter.

First, you can throw away anything where they don’t even give you a hint of what they want you to write, nor who for.  They aren’t professional.  The less they tell you, and the more hype, the more likely it’s spam.  For example:

Do you love writing???
Do you love making money???
Then this is the opportunity for you!
Internet companies are looking for fresh, new writers to create original content for their websites, blogs, and newsletters. The more articles you write, the more money you earn.
Write about almost and topic or subject you want. Write from the office, from home, or wherever…

This is obviously crap.  No specifics, no idea who it’s for.  Just ignore these.

If you are willing/desperate enough to write search engine optimized stuff, a lot of online writing leads there.  SEO essentially means marketing writing, which is probably the largest market out there for online writing.  What you are doing is writing something, but you are following some rules to work in the right keywords.  This will help the article float to the top of Google (and the other 15 search engines no one uses).  Companies get a big wood when their marketing floats to the top of Google.  If you are at all a competent writer and present yourself well, you can probably find SEO work (if you learn what it is and how it works).

Is there anything wrong with the literary prostitution of SEO? You’re asking me, the literary mercenary? The only things I won’t write for money are those which I a) am too incompetent at to even understand much less write, b) find too morally disgusting even for my rather unconventional moral code, or c) don’t get paid enough.  Most of what I turn down, that’s the reason.  The work sounds fine, but $3 an hour doesn’t cut it.  A lot of opportunity out there is designed to attract those desperate for exposure, which I am not.  I like to work with professionals who have high standards and clear expectations, with reasonable compensation for quality work promptly done.

However, I confess I got my start writing marketing stuff.