If you could read your clients minds for a day, what would you want to know?
Jef Lippiatt , Owner at Startup Chucktown
Probably information on expected pricing or price tolerance.
Overall, you can get many of these signals without needing to read their minds if you apply yourself. You can tell a great mixture of things based solely on, posture, facial expression, tone of voice, level of eye contact and the number of questions they are asking.
Never settle for a yes or no. Probe deeper by asking one or more follow up questions until you feel you've achieved the level of detail you needed.