Build a really cool marketing website, an awesome promo video, then buy google ads to test how many people click the 'sign up' button that says 'Coming soon - give us your email here so we can let you know when we launch.' Divide amount spent on ads by number of emails and you have a cost of acquisition. See how this compares to potential ad income / user monthly cost. If the latter is higher than the former, you have a quantitatively-substantiated profitable business model, and you should then talk to an investor/angel.