There are many ways to go about this, but these days the most popular method — as far as I know — is to just use PPC marketing. A good digital marketing and analytics agency can help you run various small PPC marketing campaigns targeted at various target audiences, and then they crunch the numbers to figure out which audiences responded the most to your product and/or sales pitch. Here’s an example of a company that does that: https://metriclabs.com.au/analytics-data-agency/.
This method is ideal because it is relatively unbiased. It allows businesses to find target demographics that they might never have considered targeting otherwise. For example, you may find that your line of hip steampunk sunglasses is very popular among elderly people looking for gifts for their grandchildren.
That’s a wild example, but you get the point. There might be applications and demands for your product you are not aware of, and those can lead to unexpected groups being interested in said product. PPC marketing also allows you to try variations on ads and sales pitches to help you find which ones get the most clicks, and which ones lead to a higher conversion rate.