Quality of a conference
Usually, you should be able to figure out who are the leading researchers or research groups in your research area.

Everything else is then usually fairly straightforward: just find out in which conferences the leading researchers publish their work, and in which conferences they serve in programme committees, etc. Most likely those would be the most relevant conferences for your work as well.

The same applies to journals as well.

(Note that the most relevant conference is not necessarily the same as the most prestigious conference, but it might be a good idea to start with relevant conferences... Your work might have much more impact that way, even if it does not look that impressive in your CV.)