Through blood tests. Usually this is only checked prior to surgery or invasive treatment.. Perhaps also at birth for medical records though I'm not sure. . Also, if you've ever donated blood your Red Cross card will let you know what your type is.
AB+, I'm genetically hardwired to be selfish. Also, if you want to find out your blood type it's really easy, thinkgeek.com has a blood typing kit for ~$10
Funfact: plasma is the exact opposite. So a person with O- can only give plasma to other O- but can receive from anybody, and AB+ can donate to anybody but can only receive from other AB+
Which kind of sucks for me, as plasma is a shit ton more painful to donate. On the other hand, since your blood gets filtered through a dialysis machine before being pumped back in to you, you feel amazing for the rest of the day.
Success runs through our veins ( ͡° ͜ʖ ͡°)
It's a good thing i don't have blood problems