I mean, the number terrorist attacks in the USA from white supremacist numb nuts are clearly on the increase and they appear to be increasingly invoking Trump's rhetoric, so there's that.
You can also use the FBI hate crime figures to find out who the perpetrators and victims are and compare it by year and election cycle. Racial and religious violence over the last decade or so looks like it correlates pretty well with the election cycle but had an overall falling trend under Obama up until 2015. From 2009 - 14 victims of religious and racial hate crime fell by 35%. Since that time they have increased for all ethnic and religious categories. From 2015 - 2017 the number of official hate crime victims have increased by 77% for Muslims, 57% for Jews, 41% for Hispanics, 22% for blacks, and 18% for whites. Hate crime perpetrated by whites has increased by 21%, by blacks 13%. The FBI figures only go to 2017, but
reports from city data indicate that 2018 (and so far 2019) will do nothing to reverse these nascent trends.
It's also true to say that the way the FBI derives its figures is itself likely an under-report. What counts as a hate crime differs by location as does the method of reporting as well as the desire to do so. Any %age increase in the collated figures probably hides an even more serious problem.
Now obviously this doesn't prove cause, but it does implicate Trump and the current political rhetoric of the right as a suspect in further inflaming an already endemic issue.