Do they have a LinkedIn profile? No? Redflag.
Look at their work experience. Have they been in recruiting/consulting for a while, or did they just graduate college with a degree in something unrelated like sports management? Redflag. (That one's a true story)
Is the communication personalized, or is it a cut-and-paste job description from the hiring company? Redflag.
Does the recruiter want to connect with you before even starting a conversation? Redflag.