Voice changes
Changes may occur in your friend’s voice too. Look for them and you may indicate emotional states that keep changing. People who panic have a higher pitch of voice. If their current voice pitch is lower than the usual one, this probably means that they feel nervous or fabricating lies. Without you accusing them of anything at all, they may express their suspicion.
Third party opinion
What if you suspect a completely innocent person? This is very possible indeed. If you wish to make sure that you are right, then ask someone who does not know your friend well enough to participate in the conversation and then express his opinion.