A sign of No Demand would be an up bar with low volume, volume lower than the previous two bars, and without a bottom tale. A close of the bar in the middle or on the lows adds to the weakness. This sign is confirmed if the next bar is a down bar, that means that the price has responded to the weakness.
See on the chart bellow, bar 4 (it is actually two bars) each one of them is a No Demand bar that is not confirmed, and bars 10 and 16 are confirmed No Demand bars.
If you want to learn more about the imbalances of supply and demand that drive the markets, I recommend you read VSA experts Tom Williams and Gavin Holmes, very few people in the world understand the markets like they do.
If you have more questions do not hesitate to ask. I recommend you follow my profile, I publish daily updates for the analysis of the daily chart of the EURUSD that you can see below.