A good way to determine a prospect's interest is to use a trial close. When trial closing, a salesperson is asking for the prospect's opinion about the product or service. It helps measure a prospect's interest at any time during the presentation.
Source: Sales consultant/author James W. Pickens
It's a good idea to use a trial close as soon as you spot a buying signal. There is usually no risk in trial closing.
You may get an immediate positive response or uncover some resistance that has to be overcome before you can close the sale. It's a win-win situation, in either case.