The absolute best way is to have your client do a food diary. A week is ideal, but if you can get two days, that is great. They may find that the very act of keeping a food diary means they eat less — much easier not to eat that candy bar then go into your iPhone or pad to write it down!)
If they resist, you can (if you want to be blunt) ask how committed they are to losing weight if they are unwilling to do that?
If you have a food diary from them, you can both agree on one elimination of substitution per week e.g. No wine with dinner, no eating after 7:00 p.m., adding small snacks in the afternoon, etc.)