How do you forecast demand for a new product?

How do you forecast demand for a new product?
Technically speaking, you cannot really forecast the demand of a new product.

What you could do though, is do a survey, explaining the new product, maybe test it on a few hundred people, and then check out all that data, age groups which prefer it, social status which prefer it, etc..etc.. and you would be able to get a rough picture about the general demand.

You also have to consider factors about your product, is it a durable product? or a food? can it be considered as a necessity? or an alternative to another product? (Like chicken is an alternative to red meat).

There are many many factors to consider when and while launching a new product.