No one thinks of this, until they've been burned by it. A lot of people come to martial arts out of an emotional insecurity - fear, the need to feel control in something. You'd think once they become adept in their art, they'd get over it, but a lot of instructors will simply inflict those same old problems onto their students.
I've seen instructors who emotionally abuse students, sexually exploit students, set up interpersonal drama between students to keep them from "outshining" the instructor, etc.
I look for three things to avoid this:
Do they have a mean streak? Do they have nasty insults about people? About the students in front of them?
Do they listen? Do they actually respect requests from students? Or do they simply say "try harder!" regardless of the situation?
Are they relatively straightforward about how they learned what they learned? Is there at least enough humility to admit they've received training from others? They don't have to worship a particular system or teacher, it's just that the instructors who seem to erase the fact that they, too, had to learn from others, tend to like to set themselves up on pedestals.
Basic presentation to specific advice
The best way to see how well you'll learn is to train with someone in a tester class or workshop. You can see how they present the basic material in demonstration, and how well they give specific advice to each student.
Not everyone is good at both! Which means some instructors might be able to give you the general gist well, but you'll have to figure out the smaller details as specific to you, through investigation and hard work. Other instructors might be terrible at explaining the general thing, but if you get one on one time, you can really learn a lot quickly.
Having a question or two between learning things is a good way to see how well they do the specifics - "I'm having a hard time doing X..." "I can't seem to get the power into this technique..." "This feels weird, is it because my arms are too long/short compared to my partner?" etc.
Any teacher can show you exactly what they learned. A good teacher can show you what they learned in a way that is better for you, specifically.
This means adapting techniques based on your body type (tall, short, heavy, light), injuries (knee, hip, back issues) and most importantly, being aware of what you need to prevent injuries.
One of my best kinesiology instructors also teaches acrobatics work and he points out that a lot of martial arts will demand people increase flexibility without looking at the person's body. Not everyone's bones grow exactly the same - some people literally cannot get greater range of motion in some joints without sawing part some of their bone structure. Other people may have joints prone to injury and what they need is stabilization and strengthening, not loosening of those joints.
Instructors who know their art well enough to help you train safely based on your needs, including the needs you may not have known but they can see? Those instructors are well worth their weight in gold.