Business Plans – How do I gauge a market?

I’m attempting to put together a business plan for a taxi service. There is competition in my local area for such a service. What is the best way to “gauge the market“? I will need a source of capital for this venture and I want to be as thorough as possible when drawing up my plan. I think I have a good idea (this will not be your run of the mill taxi service) but I want to be smart about assessing the feasibility of the business model. How can I figure potential earnings or the demand for this service oriented business?