So you might be wording the question wrong. The key is in "how to market" - the state of the product us irrelevant.
In other words, if you can't figure out how to reach the customers before you have z product, you're not going to be able to figure it out after you have a product.
Since marketing is harder to figure out than code it makes sense to do that part first, not last.
Then if you fail to find the market, you haven't wasted time building the product.
To answer your question; A) identify the market. B) figure out where they hang out C) go there D) talk to them