How can I judge whether an alternative "impact factor" is from an accepted and genuine source?
One way to judge a journal metric is to look at a few of the journals that are ranked highly by the metric, and judge for yourself whether they seem like important, reputable journals.  (See How do you judge the quality of a journal.)

According to the SJIF metric, this is the publisher of the US-based journal with the highest impact factor. Visit their website (if you dare - it may give you a headache). I think it's pretty clear that this journal is not an important or reputable journal. The other journals ranked by this metric are similarly unprofessional. 

Also: here's Jeffrey Beall's list of criteria that he uses for identifying a "fake" impact factor, and a list of metrics that he believes meet those criteria.