How do you know if your idea that underlies the starting of a company is feasible or not?

You need to do a market research. Is there a modern example similar to your business idea?
Is your idea so good that people never thought of it? There may be a lot of hidden challenges and those who pave it know the answer.

How can you be so sure that your target audience will use your product? Is that product truly what the audience needs? Is there other possible cause of your targeted problems? A lot of time what you think it is is not what you expect in real life. Do a quick prototype and get feedbacks to improve your product?

Is your business model sustainable? Is your product profitable? How can you get your target audience receptive to your products?

U can consult and pitch your idea with edb Singapore or venture capitals. They may fund your ideas.

To conclude, is your idea desirable, viable and feasible?

You can do a survey with your target audience to find out if they like your idea. You can consult professional bodies and talk to them about your idea.