Counting calories?

How do you do this?
I overheard some guys at the gym today talking about "I eat 1500 calories" "Oh not me I'm around 1800"
HOW do they know this?
I "try" to watch what I eat. I have 3 eggs W/ only 1 yoke for breakfast & 4 oz. of oatmeal. Usually a open face turkey sandwich (3 slices) with 1 slice of cheese for lunch & a small handfull of nacho chips. For a snack a lean cusine. For dinner a chicken breast, rice (1/2 bag of Uncle Bens) W/ fresh spinach. HOW in the HELL do I KNOW how many calories this is?

I go to the gym 6 days a week. I lift 5 days, use the stepmill 2 days for 100 floors & run 1 day (Sat) for 5K.
Counting calories?
Post Opinion