I just use a pencil. I also weigh and label each egg with its size (L, XL, J).
USA sizes:
Large eggs; 2.0-2.29 ounces
XtraLarge: 2.3-2.49 ounces
Jumbo: 2.5+ ounces
Other countries give different designations or values for the size categories. I got my information from Wikipedia.
Customers...