If they write unsolicited invitations to journals that have nothing to do with my field, or invite me to be an editor for said journals, promise rapid publication and have publication fees, I think it's fair for me to call them predatory. Its the combination of all of those and the effort of customization they put it. I got invitations from renowned journals but those came from editors that are known in the field and match my research. Predatory ones just randomly scrap titles from the web and paste them into whatever template they have for their issue of the day.