Below a number of characteristics of questionable publishers and journals are listed. Separately, they may serve as warning flags and together they form a basis for a decision not to submit your manuscript to them.
 

• The publisher's/journal's website contains misspellings and bad grammar
• The email contact address is non-professional, that is, a gmail.com or yahoo.com account
• The contact address is fake or leads to an address in a different country than stated in the contact information
• The journal is boasting with questionable and misleading impact factors (see List of Misleading and Fake Metrics)
• The publisher launches a whole fleet of new journals with similar titles within a wide variety of fields to catch as many potential authors as possible
• The journal promises an unusually fast publishing process ("too good to be true")
• They are not indexed in any well-known journal databases but they present a long list of indexes like catalogues and websites. David Publishing Company is an example of this.
• Members of the editorial board are made up or included without the person's knowledge
• They have no retraction policy which is a quality characteristic that serious publishers and journals have
• The author fee (APC - Article Processing Charge) is often quite low to lure as many as possible to publish in the journal
• Manuscripts are in many cases expected to be sent by email instead of via an online-based system
• The publisher often email individualized invites to publish in their journals
   

An email invite is often the most common way to encounter these publishers and journals

The most distinguishing characteristic of these publishers is, as has been mentioned earlier, that they send email invites that seem very personally directed toward the individual researcher which is a way assure the researcher that the manuscript will likely be accepted ("What? Do you want me to publish in your journal?" That's flattering!"). In the email, a title from an earlier conference paper the author has written might be mentioned. The publishers usually harvest these titles together with the email addresses and paste them into a standardized email message. As these email messages are usually like spam, that is, they send out a lot of theses "personal" invites, they hope that at least some will be lured to publish in the journal.

Example of an email from a questionable publisher:

A blog post about David Publishing from Leiter Reports: A Philosophy Blog: On David Publishing, Once Again--and the Philosopher's Index

In the next section, we will discuss a couple of details you could look further into and some resources you can use to assess if the publisher or journal is legitimate or questionable.
 

  Top 10 tips: How to identify fake research journals [1:00]
 

Further reading:

Cobey, K. (2017). Illegitimate journals scam even senior scientists. Nature, 549(7670), 7. DOI: 10.1038/549007a

Vence, T. (2017, July 17). Identifying Predatory Publishers: How to tell reputable journals from shady ones. The Scientist. Retrieved from http://www.the-scientist.com/?articles.view/articleNo/49862/title/Identifying-Predatory-Publishers/