What is Market Demand?
Who wants to purchase a product? what is the demand for that product in the market? how willing consumers are to spend a certain price on a particular good or service? increase of market demand directly proportional to the price. means, demand increases so the prices also increase.
But. Your question is:
How to check market demand?
Not any single-line answer. But 3 major techniques I can think about know.
Post surveys about products, industries and services on social media. Send the survey in an email. Sometimes we need to use different methods. During 1 of my startup, I visit 100s of factories for months to ask questions. I record audio of the meetings (with permission). Then I compile all data into excel sheets. This will happen when you are focusing on B2B Business service or product. For B2C, survey emails and social media polls are enough.
Don't make the complete product at the start but make POC (Proof of concept). Or make a sale on a commerce platform product to see if users are attracted to that product or not. For websites and mobile apps, we mostly use prototyping tools like Figma or Adobe XD to make an application prototype and then show that prototype to our potential users. You can show these prototypes during your survey interviews as well.
Just look around on social media and people in your social circle. Observing trending topics and products on sites like Twitter and Pinterest. Read local newspapers and other publications.
So there is not any ninja technique. What I recommend is, observing and doing surveys. On basis of surveys make your POC and Prototypes and show them to potential clients. and observe their responses again also add new observations in your experiments.