Do basic market research. Of course, look at existing services but also, more broadly, look at the target market as a whole. How big is it, who are the custome

Do basic market research. Of course, look at existing services but also, more broadly, look at the target market as a whole. How big is it, who are the customers, whether they would be willing to pay and how much, how competitive and expensive advertising in the niche is, look at industry blogs and trade magazines, basic keyword research, etc. That's just scratching the surface, you can and should obviously do a lot more to validate an idea, like dry testing.