Red Flags:
-blurry or bad pics (every phone has a good camera now, and everyone knows how to at least take a mirror selfie)
-Availabile 24/7
-Ads for multiple SPs under the same user account (usually see this on LL), but no agency/parlour mentioned or linked
-New In Town! (unless they say where they are from and have reviews in that city/are obviously touring)
-app phone numbers (middle 3 digits end in 2 zeros, like 700)
Green Flags:
-a decent website
-active or semi-active social media presence of some kind (interaction with other established providers is also a green flag here)
-a mix of professional and candid pics on ads/site
-connection to a known entity like indycompanion/merb/established agency
I agree with many of the other green flags posted, just trying not to repeat too much.