You can sometimes detect a certain tone which combined with the words used, can indicate the person probably isn't going to actually make it to a booking. Certain words and phrases in text and emails, the style, content etc also can give a feel of "this person is going to play with me" or "He's going to be difficult". Final decision is always does the person turn up and how are they in person, emails, texts, phone calls can be misleading and you can't rely on them 100%, but they do give a guide and the sleazy drawl, usually containing the words "I'm a businessman" as if those 2 words should convince me he's not an axe murderer, offering champagne or promises of lengthy 'regular' bookings also get my spidey senseys onto full alert.