How do I know if I've copied a photo from camera roll to album? Let's say I have 500 images that can be categorized in 5 albums: cars, houses, vacations, family. So I went to my camera roll, selected about 100 images related to "cars" and added them to folder "cars". Now I go back to the camera roll and I notice another picture of a car, but I'm not sure if I selected that one and copied it to the album.

Is there a way to know what pictures in camera roll were added to an album?

I have iPhone 5C, iOS 8.4.1.
They won't be duplicated. Albums only reference the pictures in your Camera Roll, they aren't actually copied.