Just like how mushroom colleges/universities are all over the place the same way conferences too will be. In order to validate and authenticate the quality, first of all, look out for the ranking of the institution. Secondly, find out who the partners/sponsors of the conference are since no reputable organization will want to associate itself with less valued programme. The third thing you have to look out for is whether the proceedings of the conference will be published in a journal or not and whether the journal is a recognized one (UGC approved in case of India) and finally don't forget about the team of the conference. Thus the relevance of the conference theme.
Two key reasons to present at conference are to get your name known and to learn more about a new or niche topic. So I would suggest a fairly simple selection process here: (a) How relevant is the event to the theme of your specific research? Does it have a close fit? (b) What is the academic reputation of the organizers (and the committee or keynotes) in the field associated with the topic and discipline(s) primarily associated with it? Look at what books and articles they've appeared and which publishers have carried them. Look at whether the have a history of successful grant bidding in the area. Look at what journal editorial boards they belong to, whether they edit a journal or book series, etc. The same goes for any list of existing speakers. (c) Next you can consider practical things like registration fee, conference location, etc. These practical issues might big influence your final decision, but it's best not to start from them. It's a useful bonus to have a guaranteed publication outlet for your conference contribution, but you have to look carefully at the quality of that opportunity. Do great work, and other outlets will take it... To sum up, if unknown scholars are charging high registration fees for events with vague titles or little coherence in terms of the topic, it's probably not worth choosing.