1. Look for someone with a thirst for knowledge and a desire to learn new techniques and try different ideas. If they are constantly reading up on new techniques and research, they are a lot less likely to just be using the same program for every person who comes to them, and a lot more likely to be willing and able to work within a person's strengths and preferences.
2. Run far away if you see a very restrictive low-calorie diet plan. Personal Trainers are NOT nutritionists. They have some basic knowledge, but should not be doing any more than making general recommendations. I highly recommend searching out a Registered Dietician in your area for sound nutritional advice. You can search for Registered Dieticians in Canada at http://www.dietitians.ca/Find-A-Dietitian.aspx.
3. A good trainer will correct sloppy form and have recommendations to avoid injury whilst exercising.
4. Find someone who has time for you. If a trainer is fantastic, but they don't have time to answer a few quick questions when you're struggling, it's pointless to have one.
5. Anytime someone is constantly trying to sell you on a particular product or gimmick and is unwilling to make a general recommendation and let you choose your own product is probably a sales rep for that company. This does not necessarily mean it's a bad product, but it doesn't mean it's a good or valuable product, either. I find this to be a breech of trust between client and trainer.
Really, you want a trainer who has passion for what they are doing. You can definitely tell if it is "just a job", or if a trainer is truly passionate about helping others. And, to be honest, a bad trainer can be a lot worse than no trainer.