Market Analysis - there are many books on entrepreneurship (for example - https://goo.gl/tyJSu1) for education tools you can talk to people who will use them - teachers or students, and try to estimate how many of them, and how much they are willing to pay, or if you can make money off ads.
for your example project, you send a to teachers at local community colleges and try to get their feedback and maybe they will recommend it to schools. Also, you can post the link to your project at language learning online forums or meetup groups.
I mean, not every side project will be big business or even business at all, but I think about it as a good practice if you really think about entrepreneur path.
I myself went back to coding for now, I enjoy it too :)