Always trains a certain "type"
Now, don't get me wrong -- it’s normal and good for trainers to specialize in specific types of training. For instance, there are prenatal and postnatal specialists, cardiac specialists, and cancer specialists. If you have a specific condition or need, it’s important to seek out a trainer who has a background in the area.
That said, if your potential trainer's "type" is busty blondes or rich single men, you might have cause to question his or her priorities. You want a trainer who’s focused on you for the right reasons: helping you achieve your health and fitness goals. Sadly, some trainers are more interested in what their clients can provide them, whether that’s intense sexual fantasies, business connections, or big tips.
Has the demeanor of the world's angriest drill sergeant
Again, it's completely normal for trainers to push their clients. Frankly, most clients need it. If they didn't, they probably wouldn't hire a personal trainer.
But it's not normal for a trainer to be angry, abusive, or overly zealous about pushing you to the point of serious physical pain. You'd be surprised by how many trainers prioritize their own egos and the design of their workouts over their clients' needs. If you see a trainer continuously pushing clients to do more, work harder, and go faster to the point of pain, poor form, or injury, run as fast as you can in the other direction.