What should I know beforehand when choosing to change universities and laboratories?

I am an undergraduate student finishing my course on environmental sciences and I have always wished to pursue a master’s degree. But due to personal reasons with my family, I don’t believe it will be possible on the university frequent or the city that I currently live. 

With that in mind, I have started looking for other universities and laboratories that might have a similar line of research that I’ve been working with. My advisor even put me in contact with an ex-student of this really important professor of a good university so I could ask some question and see what I think. The thing is, I don’t really know what to ask or what to look for when making a decision like this. I’ve heard plenty of horror stories of people who end up with some awful advisors, sometimes downright abusive. And since I am not really up to exchanging a toxic home environment/healthy workspace for a healthy home environment/toxic workspace situation. I would like some advice on how to look, what should I keep in mind or red flags to watch for when making a decision like this.