I don't think it's true. Take the reverse case: can you tell that an idea is bad without executing it?

I agree that there are idea for which there are obvious reasons that the idea is bad but most of the time there isn't that certainty.

Many successful companies such as AirBnB or PInterest had a hard time raising money because investors thought those were bad ideas.

On element of a good startup idea is that there's little direct competition. If the idea is obvious there's usually competition.