I occasionally receive emails inviting me to present in a predatory conference. These are usually easy to identify, given that they call me a "Dr" or a "Prof" (only a PhD student at the moment), never heard of them (6th conference on something.. What were the first 5?), weird English, bizarre locations, organised by an organisation outside the field, etc.
When you're publishing a predatory journal I guess it's not too hard to actually publish the journal eventually. Just put some PDFs on a website. What about conferences? Organising a conference is harder: you have to arrange a venue, audio/video gear, food and a bunch of other things. Let's say that I do end up submitting and paying the abstract fees. Is there an actual conference happening somewhere?
EDIT - let me focus this a bit..
- Is it possible that one signs up to such a conference, arrives, and then there is nothing on site?
- Are the (sometimes famous) keynote speakers aware they they are listed? Do they show up to the conference?
If there is an actual conference, and people end up attending it, what is predatory about it? Eventually, it's a meeting of people discussing science about a particular topic.
So far as I can tell, and have read online, predatory conferences typically are associated with an actual event.
To my mind, this seems to be a key difference between "predatory" and "fraudulent." If a conference takes your money and no event is organized, then there is clearly false advertising and grounds for legal action, perhaps even criminal charges. If a conference takes your money and organizes a horrible, worthless, CV-staining joke of an event, on the other hand, there is no regulatory body that you can complain to: scientific quality is considered to be a matter of judgement, and there is no way to easily distinguish between an honest-but-poor-quality conference and a intentionally predatory conference.