How does everyone know their blood type?

Everyone seems to know their blood type.

I get blood work done regularly for a medical condition and also for yearly checkups, but they have never included a blood type test. I asked my doctor what mine was, and it's not in my chart and she said it wasn't standard to test for unless indicated.

So how does everyone find out what blood type they is?

Edit: I would love to donate blood, but I have a medical history with a specific cancer that makes me ineligible.