How do you judge the potential of a newer journal? I'm in a fairly niche field that is still in the process of organizing itself. As such, there are a number of existing professional societies that are spinning off journals along with newer societies that are getting into journal publication. Since some of the usual metrics for judging a predatory journal may not apply, how do you judge the potential of a newer journal so that the publication does not end up being a wasted effort?
To expand upon part of Dave Everitt's answer,


  
  look at the founders of the journal - did they start it primarily as a platform for their own publications, and do these authors and their associates make up the bulk of the authors?
  


I would also look at who is publishing and editing the journal. 

Is the publisher affiliated with a scientific society? Sometimes scientific society will launch new journals (e.g., the Ecological Society of America launched Ecoshpere to be their open source journal).

Or another is a new journal from an established publisher? Established commercial publishing companies are launching new journals. Although people argue about the ethics of publishing with them (cf What are some examples of negative effects on a career for boycotting Elsevier journals?), my own experience has been that their products are usually scientifically sound (presumably because they are afraid of hurting their brand image). Additionally, journals such as Nature now have spinoffs (and spinoffs of spinoffs, e.g., Nature to Nature Methods to Scientific Reports) to catch paper that do not fit into their flagship journals. 

Also, who is the editor-in-chief and other editors? Do you know them in your sub-field? For example, when the journal Theoretical Ecology launched, the they had Alan Hastings as the EIC, a highly respected Ecologist.