How to tell a double yolk egg without opening it?

[upcdayz]

When my hens started laying regularly, they would occasionally lay a double-yolked egg. I only found that out when the egg was opened, but I've seen several posts from folks who can tell a double-yolked egg without opening it. How can you tell an egg has two yolks without cracking it open?

 

[FireTigeris]

Quote:
it will be very large, also you can candle it if its light enough in color or you have a good enough light.