Checking-in during the conversation, especially if you are conducting the demo via web/conference call, is very important.
Ensures the prospect is engaged in the conversation and paying attention. Conference calls can be challenging. A prospect may be checking email or focused on other activities and NOT listening, especially if there is too much "telling" or feature dumping.
Allows you to identify additional qualifying information. If you check-in with a question, the response may give you additional insight into the company, the opportunity, etc. For example: "Is what we are showing you in alignment with what you were anticipating?" or "Based on what we have shown you so far, I think we are in alignment with your requirements, do you agree?" Their response will provide the clues that will tell you to either stay on track or move in another direction.
When you respond to a question, always confirm at the end "did I answer your question?" You want to be sure you clearly understood the question and provided enough information to satisfy the prospect.
Demos are a two way conversation. When you just "show up and throw up" - you pay the price. You miss the opportunity to further qualify the prospect, address objections and set the stage for the next step in the sales process.