Which is better at verifying that a conference is reliable: Scopus, IEEE Xplore or DBLP? In the case that I got one paper published, which of the aforementioned repositories have more relevance (so that I can determine that my work has not been published in a nobody-knows-about, fake conference)? I have read that some papers that are in IEEE Xplore belong to bogus or fake conferences and there is also the same problem with Scopus. I have also seen that in some occasions Scopus miss some citations of my articles, while for example in Google Scholar they appear. 

Well the bottom line, which repository is better viewed for backing up that one publication was in a reliable conference?
Anyone who is reasonably experienced in your field (i.e., anyone who might hire you) will know which venues are "relevant".  They won't base this on IEEE Xplore, Scopus, or DBLP, but on their own knowledge of the field.

Hopefully, anyone who would hire you will also know enough to evaluate your work directly to some extent, rather than relying on the reputation of the venue as a proxy for the value of the work itself.