How do you test a new service/product idea?

If I have come up with what I perceive to be a new product/service how can I go about testing the viability of the idea?

I could spend the money to build it first but that could be counter productive.

Of course no idea is genuine however this idea is being exposed to a new market and thus in my opinion is marketable, scalable and profitable?

I am not afraid that the idea will be stolen but need to conduct enough research to know whether it will work in this field for sure.

As an example I am building a wordpress based facebook fan page creator but have integrated some really 'out there' features that will make it different from the rest. However I know that it will be profitable since we have seen WSO and other launches related so well.

This new idea however I have not seen anywhere done they way I want it and makes me think either it is not what the market once or something the market has yet not thought about and consequently I have a potential business model. Again I opt for the later since it is a 'valuable' tool/service that a lot of people can use and 'solves a problem'.

Any thoughts?

Since it is IM related is it worth me asking people here? Maybe in private or something.