Create a simple landing page describing your service and include a sign-up field to notify people when it becomes available. Then, run an AdWords campaign on Go

Create a simple landing page describing your service and include a sign-up field to notify people when it becomes available. Then, run an AdWords campaign on Google targeting keywords relating to your service that people may be searching for. You'll get a decent idea of interest by assessing your click through and conversion data after about a month.