For many reasons it's always difficult for one person to say if another person is racist. One of the biggest reasons is because the word itself is effectively undefined. It can mean just about anything that will help you prove your point at that particular moment.

Take the Trump/Curiel incident for example. In your post you mentioned that "discrimination" was a necessary component of racism. There was no sign of Trump discriminating against the judge. In fact, I don't know if it would even be in Trump's power to discriminate against him.

The other popular definition of racism is "power plus prejudice". This is even more problematic is you are going to accuse Trump of racism. In this situation Curiel is a Judge, and he explicitly has all of the power. In fact, if you want to look at it in that way, it is Judge Curiel who should be defending himself against the charge of being a racist. He has the power and he was being accused of being prejudiced against Trump, largely based on Trump's race.