How to identify predatory publishers/journals
First, you should probably publish in the same venues that you read and cite.  Presumably those are reputable.

Now to describe low-quality vanity publishers.  Two essential characteristics are:


The publication of very low quality material.  This is usually immediately recognizable to any expert.  Sometimes it's obvious to anyone; for example, read this abstract.
A business model in which the author (rather than the reader) pays the publisher.  Of course, this by itself isn't necessarily indicative of a low-quality publisher (think PLoS).   But low-quality publishers can't make money off of subscriptions, since they provide no content of value.


Additional common characteristics of such publishers are:


Mass e-mails (spam) to academics, especially when the recipients include researchers in unrelated fields.  These e-mails may request submission of conference presentations, papers, or book manuscripts, or may contain invitations to journal editorial boards.
A high number of prominent typographical errors in text attributable to the publisher.  For instance, at the beginning of this article "abstract" is mistakenly spelled "abstarct".