help with starting a business, have an idea that is a twist on a popular franchise

Hi guys, I have what I think is a really good idea for a restaurant concept. It is similar to a franchise that is already successful but I put my own twist on it.

I currently have an office job that pays the bills and is in a very stable sector. It just doesn't excite me that much and I am not passionate about it. I know some people have the mindset that a job doesn't have to be your passion, because working a job is what enables you to fund your hobbies after work hours. But I really want to work in an industry where I am passionate. I enjoy cooking, have worked in a restaurant all through college, and generally enjoy the hospitality industry. But I ended up taking a major in college that was the "safe bet" (accounting). My questions are:

1. How do I know my concept is viable?
2. Would a good idea be to set up a small pop up shop and sample the product to see if people enjoy it? Or to test with relatives and friends?
3. Is the competition too fierce going up against huge franchises? I don't necessarily believe my end goal needs to be franchising, but my idea is sort of geared towards that.

And I know it's just an "idea" now, nothing more. But I feel excited about the potential.

Thanks for any insight!