How to be a good reviewer when suspecting dubious results or being sent contributions outside my field of expertise?
In any field, once you start reviewing, you're going to see a lot of problematic submissions, because there are a lot of people out there who submit really bad manuscripts. In some cases, the authors are unaware of their problems, while in others they're trying to slip a minimal paper past peer review because they aren't really interested in the science---they just need the publication to graduate or get promoted.

Your job as a reviewer is to make sure that what gets published is credible.  That means that if you don't understand something or don't feel that the evidence presented supports their conclusions, you have a duty to report that as a problem with the manuscript. Depending on the severity of the problem, you might recommend either revision or rejection---and the handling editor will make the actual decision.

You shouldn't, however, demand that authors to do new work just because you think it would be interesting. You also shouldn't expect to know every tiny detail of their methodology.  So, where to draw the line?  Here is how I think about it:


The authors need to provide enough methodological detail that no significant implementation decisions are missing.  
The conclusions need to be well-supported by the evidence presented.
The overall thrust of the paper should be interesting to a typical reader of the journal (this one varies highly by journal).


None of this will allow you to detect clever fraud, but not-so-clever faking often leaves a clear trace (data that looks too nice), and cherrypicking of data is usually detectable because the authors present data from very few samples or trials.