Then when you get home and really figure it out, you will either be happily surprised that you didn't over indulge. Or you'll be glad you stopped when you did!
I went to the fair, and HAD to have a fried artichoke heart sandwich. I didn't want to stop and use my phone to look up every ingredient, so I just threw in a fried chicken sandwich with sauce figuring it would be close. Then I felt guilty about the 600 calorie lunch, skipped the beer stand, drank lots of water, and avoided the curly fries all together, but did give in on a free scoop of ice-cream
When I got home and actually dialed in all the ingredients, the fat and calories were WAY less than I had estimated and I was able to stay within my RDI for the day. The fat was still a little high, but hey, I only go to the fair once or twice a year!