As it has already been said by @jakebeal here are good CS conferences not indexed by Web of Science (WoS) or Scopus. But there are even more complex issues regarding how to evaluate the quality of a CS conference using these bibliographic indexers.
For journals it is generally agreed that WoS is more restrictive than Scopus, and thus journals indexed by WoS "should be better" than the ones only indexed by Scopus. For conferences the things are less clear. I dont know the inclusion rules for conferences in WoS, but one of the reasons why conferences "that appear in WoS" are not that great is that WoS indexes the "Lecture Notes in CS" as one of its journals, and usually the conferences that publish their proceedings in the LNCS are not top of the line -they are usually regional or very focused conferences. But notice that there conferences are not themselves indexed in WoS, but their papers are!
As far as I know DBLP does not make any evaluation of the quality of a conference to include it or not in its database besides some evaluation whether it is a CS conference or not (and I dont know how they decide that). DBLP also has some bias, a lot of conferences in different subareas of CS are not in their database (originally it was an index of publications in databases (DB) and logic programming (LP) thus the name).
ACM DL and IEEE Xplore are also complex because they include the proceedings of not only conferences sponsored by the two associations, but also conferences that made publication agreements with them. In particular IEEE seems much less selective than ACM regarding these agreements and there are very likely really bad conferences indexed in IEEE Xplore.In particular, there was this paper that identified more than 100 fake/computer generated papers in the IEEE Xplore database - these papers were published in the REALLY BAD - STAY AWAY FROM IT kind of conferences and they are/were in the IEEE Xpore. I think that ACM is probably more selective but I am not 100% sure (at least no one discovered fake papers in the ACM DL yet).
Before I get to Scopus/Scimago let me discuss three reasons to ask the question you asked.
a) because you want to choose a conference to submit a paper to
b) because you want to know if you can "trust" a paper published in one of these conferences you read
c) for after-the-fact evaluation purposes - you want to argue that the paper you published in one of these conferences should still be counted even though the conference is not indexed.
I will not expand on the c) line. As for the a) line - choosing were to submit -- there is no substitute for the experience of more senior researchers, unfortunately. There are subareas of CS that have large conferences with many topics, and the good ones among them will probably be indexed in Scopus (and IEEE Xplore or ACM DL). But there are subareas that hold very focused and small conferences and workshops and you need the experience of a senior(er) researcher to know which are the good and the not so good ones.
As for line b) "can you trust the paper for this conference" I, myself, like Scopus/Scimago (I know that Scimago uses data from Scopus but I dont know the details). If I find an interesting paper though Scopus I tend to trust it, more than if I find it only though Scholar.