1- Unusually bad English (Not talking about normal immigrant English like mine, but like really bad, no way someone hired this person to recruit for these kind of positions). 2- These `recruiters` talk in very low volume, I had to tell them to speak louder, my guess is that they are in an office with a lot of people doing the same. 3- They ask you about weird stuff in their e-mail communication, like sign something in early phases of interviewing. 4- When tell you when they will call you and the number is flagged as 'SCAM LIKELY' then they tell you to save the number, super red flag
After that I googled the companies / job postings and were super generic with bad reviews in glassdoor.