When the day is done, and you've got all your calories and activites put in the logs, then you can go to the 'Overview' tab. It gives you the calories for the day with pie chart for the macronutrient breakdown. It also gives you (at the bottom, on the right) calories in and calories out (burned). You can see there if you used up more calories than you ate.
These tools may not be absolutely precise (you'd have to be in a lab for that), but they are pretty good indicators. I find that when I'm not using up more than I'm taking in, calories are the reason I am not losing one week. I only weigh once a week because the normal fluctuations you see every day are just too confusing!