If the subject's eyes narrow, you are probably guessing wrong. If the eye lids open a little wider, you are probably right. Clothes and body posture also reveal information. Some may talk to the audience before hand, or get someone to mingle with the crowd to find out information to use later. Simple research can be helpful.
Most people do know someone with medical problems.
Play the percentages. John Edwards uses these techniques to gain information about a person, or the dead person they are concerned about. Faith healers do this. Peter Popoff had his wife go out into the audience before hand to gather information, then tell him the information with a little radio receiver in his ear. People think God is giving him the information.