When assessing ideas make sure to show proper respect to the proposer.
Consider the following two examples:
- How will this ever work? Your numbers don't make any sense whatsoever! How did you calculate this? Do you even know basic mathematics? And why would anyone buy this when it already exists, and has been done may times over? This is a stillborn idea, and I feel like I've wasted my time just by looking at it.
- I'm afraid that I don't understand who are the target customers and what would make them buy this product. The numbers don't really add up, perhaps there was a mistake in your spreadsheet? There are few companies out there doing a similar thing, how about you look a bit closer into what makes them successful and use that insight to revise your idea?
Go hard on the idea itself, but be gentle with the person.
Offer candid feedback, devoid of aggression and accusation.
It's that simple.