I think that if there is a significant amount of alcohol, or if the alcohol is uncooked, it is on the cook to say something.
If the eater cannot have any alcohol pass his/her lips, period, then it is that person's responsibility.
For example: I made a champagne turkey for Thanksgiving. I used one bottle of champagne over a 13lb turkey and then used the drippings to make gravy. I did tell my guests that there was champagne; chances are it cooked out, but there was a significant enough amount that it warranted a mention.
If I used a quarter-cup of white wine in four cups of sauce, I would probably not think to mention it.
If I made alcohol-filled truffles, I would mention it because the alcohol is most definitely there and is not cooked.