I don't know about "scary looking clients", as posted by others we should never judge a book by its cover but there are some "warning behaviors".
Most working guys simply let their 6th sense guide them or they learn as they work. PNP, bossiness, texting late at night sounding like a teenager( u free now) , endless texting, bargain hunting, sounding desperate, asking the same questionnaire over and over again, etc. might be a sign of a bad client.
Unfortunately, the definition of "scary" might have some social bias.