Greg Williams, author of Body Language Secrets to Win More Negotiations, says to watch for body language signs that indicate doubt during negotiations. Signs include: perspiration on the brow; excessive fidgeting; fondling objects; and constant rubbing of the neck, hands, legs, etc.
Those actions often indicate uneasiness, stress or a lack of direction about what to say or do in response to your comments. When you see those signals, you likely have the upper hand. You may want to make them sweat a little more to reveal your power in the negotiation.
However, most likely you want to offer them some sort of reprieve to ensure that the negotiation continues. If they become too uncomfortable, they may back out altogether. Instead, give in a little now and ask for a concession in another part of the negotiation to keep the discussion moving forward.