You need many qualities to become a trainer. But for kettlebell training in particular, the following three are very important. These are the things you should look for in a trainer, or if you want to become one then you should develop these qualities.
1. Kettlebell technique
This is of paramount importance. You can't hurt the client with poor technique. If all the other pieces are in place but this is off then nothing else matters. You have to be able to demonstrate proper technique in order to teach other people how to do it. And I don't play politics here. Kettlebell trainers love to argue minutia about techniques but the important thing is client safety. If the client is safe then the technique is likely valid. Whether or not a world champion uses a different technique is irrelevant.
2. Work capacity
This is a big pitfall with trainers. Just doing a few good reps with the kettlebell isn't enough -- you need to be able to do hundreds of correct reps straight. For male trainers, there really shouldn't be anything you can't do with the 16kg kettlebell. You should be able to swing it for at least 5:00 straight and do 100 snatches, cleans, etc. This is the capacity that the kettlebell trains. We're all about strength/endurance and trainers should have that quality. They should be able to show solid results with the kettlebell. An "I can't do it today" attitude isn't going to cut it. Kettlebell skill is about what you can do EVERYDAY, not once in a while when you feel like it. I can tell right away from just a few moves who really uses the kettlebell and who doesn't. Look for a trainer that has trained themselves to use it well.
3. Programming ability
I can change my workouts on the fly and you want a trainer that also knows how to program a workout given any limitations or special circumstances. BTW this is probably the Holy Grail in kettlebell fitness. Programming isn't widely taught or understood. It's an art form and few master it. I've learned through trial and error what works and doesn't. Beginning trainers can rely on programming by others but they should be doing their homework to understand WHY and HOW that program works. I can look at a program and usually understand just by looking at it what it's supposed to achieve and for whom (assuming the program is clear). You ideally want a trainer that can program that for you.
So use these three attributes as a guide and you can't go wrong.