The first thing to check is what the index is claiming to measure, and how it is trying to measure that. Due to it's name, the Scientific Journal Impact Factor appears to be similar to the traditional Journal Impact Factor (JIF) from Thomson-Reuters. From its name you would therefore assume that the SJIF is measuring some sort of average citation rate of papers published in the journal. However, the list of evaluation critera of SJIF doesn't include any information about citation rates. It also doesn't mention a database of citations, which is needed if you want to evaluate journals based on citation performance. In contrast, the JIF from Thomson-Reuters is using citation information from the Web of Science database while the SCImago Journal Rank (SJR indicator) and the Source normalized impact per paper (SNIP) are using the Scopus citation database.
In fact, the evaluation critera that SJIF claim to use includes just about everything except information about the citation performance of papers. The index is therefore very misleading, since most people probably read it in comparison to the normal Thomson-Reuters JIF. Instead, they claim to evaluate such things as the Editorial Board, print quality, Internationalization, indexation, review quality etc. It is however completey opaque how all these components are weighted to produce their index. The minimum evaluation critera of SJIF also includes that; "At least one issue [and] At least 3 articles must have been published". To me, this shows quite clearly what kind of information their evaluation is based on.
Having said this, I also agree with the other answers that an easy check is to look at the score of well-known journals in your field (if they are ranked at all), to see how they are ranked in comparison to other journals.