It's something you develop a feeling for. If there's lots of direct eye contact, smiling, seeming genuine interest (from body language as well as verbal), then it tends to cut down on the chances of problems later. There are no 100% certain answers. Even very experienced guys occasionally have an issue. But for the most part, I stick to reading body language more so than verbal. Are they looking at your face when talking, do they angle their body towards yours or away, do they lean in to you or away, do they make any physical contact... little things that add up to a bigger picture.