I need my case number to track my repair?

I have an iPhone 6 that i recently broke and I have sent off my device to get fixed. I rang apple support and they scheduled my phone to be collected. When the box came that i was to put my phone it said that i could track my repair and see its progress but i can't remember where i wrote the case number. Is it possible to find out the case number again so i can track my phone repair?