First, how about these for ground rules:
1 - Assume good faith in the other person unless they give you reason to do otherwise.
2 - Don't explain other people's experiences to them.
Then, when you hear someone's complaint about racism they have experienced, and you've listened with an open mind and thought it, your response will be one of a possible range of responses. I'm going to list them. See if you agree.
Starting with zero:
0 - The described experience is in the range of normal things that happen to people from time to time and is really not worth mentioning.
1 - The described experience is not very nice. The complaining person should try to put it behind them.
2 - The complaining person definitely has a grievance.
3 - An event amounting to a hate crime has happened and somebody needs to [lose their job, go to prison, etc.].
Working backward from zero:
-1 - The person is seeing racism where it doesn't exist. Could be an honest misunderstanding or just someone wanting attention.
-2 - The person is outright lying about what happened.
-3 - The person perpetrated the hoax they are now complaining about, and should now [lose their job, go to prison, etc.].
Does that about cover it?