What’s the best way to test out an app idea

I've got a client who wants to design and develop an app where you can build personal profiles as an artist in the entertainment industry. As they are from this industry they have a lot of background knowledge and experience about it and are sure it will be successful.

My question is how should i go about testing their assumptions and fine tuning the user flow before designing or developing the app. right now i'm thinking to do a low fi click thru on Invision and test it on some of their target users. Are there any other ways i could test it out?