1. Approachable. When you're first looking around the gym for a spotter, the person you look at needs to be approachable. If they have their headphones on and are never really talking to or acknowledging anyone, they might prefer to be left alone.
2. Strong. Are they strong enough? The main function of a spotter is to be able to help you if your muscles fail. If they aren't strong enough to get the weight off you, find someone else. How can you tell if they're strong enough? Mostly by looking at what kind of weight they lift, how often they go to the gym or even by asking if they're comfortable with the weight you're attempting. Being big isn't always a good sign of strength. There are plenty of smaller guys who are stronger than they look.
3. Experienced. Is the person experienced enough? How often do you see them at the gym? Are they doing exercises that require advanced knowledge or are they always on the treadmill and ab machines? If they never do any of the exercises you're trying to get a spot for, how are they supposed to know what kind of spot to give?
4. Listen. After you ask someone to spot you, do they listen to the directions you provide? As selfish as this may seem, spotting is about you, your safety and your workout. If the person can't adapt to how you need to be spotted, they might end up impeding your workouts.
5. Feel Comfortable. And finally, you need to feel comfortable having this person spot you. If you've ever done a heavy set on your own, you know the feeling of thinking that you might hurt yourself. This doesn't help if you're trying to push your body. If you have that same feeling with the person who's spotting you, they aren't helping you at all. Spotting is about mental as well as physical help. You need to feel secure to be able to push yourself past your limits.