1. Go to google keyword suggestion tool and check how many people are searching for something you are building. Say, you are building Employee Evaluation Tool - close to 74K searchers are made every month in US and 135K globally. Competition is medium, so there is still some space for new player. So, yes there is some demand for problem I m trying to solve.

2. List down top 10 blog post links which talk about Problem your product will solve and post comment on all those blogs in 3-4 lines : first two lines about the post, next two about your project & link. So, you will start getting some traffic atleast say 20-30 users daily from all those 10 blogs combined.

3. Start creating good relations with people who have given you these emails. Tell them something like : "Hi, Thanks for giving me you email. I am building this product where you will be able to choose pre-made Employ Evaluation forms or will be able to edit them too. What more do you think you will need to solve your problem of Employee evaluation. Thanks."

So, with in 2-3 weeks, you will have really good idea if this thing gonna work or not.

@sharmag88