As for red flags for me, let’s see…
…freshman classes over 300+ students where you type in a code for attendance. Not good for either of my kids.
…one prestigious school had their student panel talking only about how hard all their classes were and their daily stress. A few had considered transferring. Not a good selling point.
…how many kids on campus are actually wearing their school sweatshirts or logos
…dorms that are dark, hidden by overgrown trees, worn down or out of the way
…at one school they told us it was a state law that they could not show us inside a dorm. (We had just seen dorms at their flagship?!?) Then my daughter pointed out to me the bars on the first floor windows 
…I like to ask the tour guides which other schools they were accepted to and why did they choose this one. One tour guide told me this was her last choice school but her dad promised her a car to attend bc it was the cheapest option for her.
…if you have time go into the local restaurants and stores near campus. If you tell them you are considering that school they will tell you their honest answer. At one school a shop owner told me that honestly she would not send her daughter there.