How do I know if my idea is a good idea?
To be honest, that question is irrelevant. It's not the idea - its what you do with it that matters.
Case in point: The famous novelist E.L. Doctrow wrote a book called "The Waterworks".
Set in 1890's New York - The Waterworks deals with a series of grisly murders around the Croton Reservoir. There's a lot of political intrigue and the protagonist is a crusading newspaperman.
At about the same time, a little known writer named Caleb Carr wrote "The Alienist".
The Alienist is also set in 1890's New York City. It also deals with a series of grisly murders around the Croton Reservoir. And yes - there's a lot of political intrigue and believe it or not, The Alienist's protagonist is a crusading newspaperman.
Two identical books? They should be the same, right?
Hardly. The Waterworks is as boring as Hell. The Alienist is a gripping page turner.
Same idea. Same setting. Same time period. Same characters.
One is a failure, the other is a brilliant success. <-- my opinion - but you're welcome to check out the reviews on Amazon.com. I'm not on the minority.