How can you tell if someone is racist?

I was at wal-greens after playing football the other day to buy some gatorade and water for my friends since we were all tired after playing and the cashier lady, some white woman old 30-young 40ish gave me a dirty look and handed me the bag very rudely( like she gave me the bag pulled away real hard and looked at the white guy behind me smiling like if that didnt happen) Im 17 and latino