Calorie counting: How do you do?

I mean, when I buy a sandwich, I don't know how many calories it contain. Not even roughly. When having dinner, I don't know how many grams of potatoes are in my plate or how much meat is on my plate. Do you guys weigh it?

I certainly don't know how much "a serving" is, and I do suspect that if my idea of a serving size was normal, I wouldn't need weight loss in the first place.