Ask, “How far along the decision making process are you?”

When someone comes to you asking for feedback or suggestions about their idea, you need to know something before you offer your thoughts… How far down the decision making process are they? How personally committed are they to this idea?
If they are 90% decided and you start quibbling about details! you’re only going to annoy them. Especially if this is your boss.
Knowing how much they’ve already “made” their decision greatly affects how your shape your response to them. It opens the door for a win-win as you give them the type of feedback they’re looking for, and you from there you can start to question deeper things about their idea.