I find a conference BDAS (bdas.polsl.pl/default.asp). Noticeably, almost all the papers published in the previous conference proceedings are from the host country . Can anyone give some comment on its validity and level? Another question is how to verify that sponsors listed on the website really sponsor it? Thanks!
You have placed several questions on Academia.SE of the form: is X a good conference?
There are several ways of finding out: one is to simply ask Dr. Google, see if it's on Beall's List or just google predatory or ranking to get a better idea. Still not sure? Google some of the organisers' names. If they're big-name people, then the conference is likely legit.
As a general rule, if you need to go and ask on Academia.SE whether the conference is any good, after getting inconclusive answers from doing the above, the conference is likely not very good. Good places are, well, well-known.