Did you know that crested geckos shed their skin? They often do it when you aren't watching such as late at night. Crested geckos will eat this shed so you can't really tell unless they leave evidence.
(Shown below are pieces of shed in a lay box.)
A healthy crested gecko shed should come off fairly easily. If your gecko has stuck shed (shed that didn't come off easy) it can be because there isn't enough humidity in the air or if its kept too wet it can stay stuck in to the skin. It is important to get dry shed off safely because stuck shed can cause problems such as cutting off circulation. Wet shed can cause bacterial infection which can lead to death.
You can do something called a sauna to get tough shed off. Place your gecko in a deli cup with holes and a moist paper towel. You do not need to over saturated this set up, the shed should easily come off. If the gecko is too wet you can dry not spraying the enclosure too much!