A quick test of a decent idea is trying to sell it. Just go to the mall and see if anyone will buy based purely on your pitch and passion. If your ideal customers are big companies, pick up the phone.

The truth is that 99% of the big companies didn't start off the way they look today. The key to quick decipher the parts of the idea that are interesting is to find a customer, and try and sell them the solution as you describe it.

So, I actually don't think there's good or bad ideas, only ones with super small Total Addressable Markets (ex: of 1, you, to solve a problem that only you care about). But even at that, if you're passionate about the idea, and you end up being the only customer, at least you've made your world a bit better - and that's awesome!

To fully answer this question: I don't decided what a good / bad idea looks like, I just execute and keep learning if there's a "there there" and will I be passionate about the opportunity.

If there isn't, I try something else.