Indications of predatory publishing:
Email invitation to submit, especially after presenting at a conference
Advertises extremely fast time to publication
No information or inaccurate information about editors and editorial board
Contact only through online form, no direct email or street address
"Indexed" only in search engines, rather than curated resources like subscription databases or Directory of Open Access Journals (DOAJ)
Published articles don't adhere to stated scope of journal or scope is so broad as to include most topics