How do I check for proof of purchase on my phone? Bought my iPhone 8+ online Apple store.

My home button cracked and doesn’t work at all, it is heating up and I checked for my serial number to see if I a warranty on it (I bought my phone online from the apple store) in March of this year, it hasn’t been a year yet, (7 months) I still have a warranty to repair my phone. How do I check for proof of purchase receipt? When I bought my phone online from Apple website — I checked my email and I can’t find anything?