It does depend on the situation.

I havent noticed racist comments with my friends, but there is stereotyping about drug abuse, money, and stereotyping of groups of people according to their 'class'. If I know they are talking bs, then I will pull them up on it.
Racism in public, I let pass unless it is directed at me or if my children question it. Then I will make a comment about how wrong it is. More so though if we hear something out in public we will respond to the comment by teaching our children that what was heard. I do the same when waiting at the lights and someone walks on the red man; "Now we wait for the green man because that is the safest time to cross the road." that gets some looks, so with racism we say "now its not nice to call people names because it hurts their feelings, and we dont want hurt other's feelings. Also if we dont know what their outfit is called, then lets ask"

With family I set them right, but now my girls will tell people if they are being rude and offending them by saying nasty things.

Im not interested in changing strangers, I want to bring my girls up to not tolerate racism. My hope is that most of us, if not all of us will do this then the tide will change small steps, but a step in the right direction