I submitted my paper to a conference. Based on a previous accepted submission, the conference response seems suspicious, not to mention that gmail marked their email as "suspicious mail". Instead of getting the reviewer's comments, I got "your paper acceptance rate".

According to IEEE, the previous book series of that conference appears in their digital library. But as far as I know, not all conferences in digital libraries are indexed.

Is it done through ISBN search or how to can I figure out if a conference is being indexed?

According to a related asked question How can I determine whether a conference is reputable?

The publisher or the sponser is known. The point is I have already participated in a conference (MIUCC) that was neither indexed nor scopus, however, the conference proceeding book was inserted on ieee digital library. So being in ieee digital library does not confirm being an indexed confenrence.