Generally, you should be able to trust your decisions, but if you have an idea that you think would be a great service (a good indicator of this is real life need -- if you needed this thing, chances are someone else wants it too), you can make something like a landing page with a short walk through on how it works (or how it WILL work) and throw that out there, post it on the relevant forums and places, and see what comes of it.
OF course, if it's something that YOU personally need, you using it is the best indicator