First and foremost, you need to make sure that the trainer you choose holds a current, nationally accredited certification in the fitness field. Some of the certification organizations that require a good, solid minimum knowledge base to earn their certification are the "ISSA" (International Sports Sciences Association), the "ACSM" (American Collage of Sports Medicine), the "NSCA" (National Strength and Conditioning Association), and on a somewhat lesser level, but still an acceptable certification is the "ACE" (American Council on Exercise). Some others like the "NAFC" (National Association for Fitness Certification) or the "NFPT" (National Federation of Professional Trainers) are a lot less strict for their certification requirements.
While it is very important that the fitness trainer you choose holds one of the better certifications, it is certainly not the only thing to look for when choosing a trainer. There are a few other things you will want to be sure of to ensure your satisfaction.
One thing to look for is whether or not your fitness trainer has a plan to suit your individual needs. Some trainers get caught in a rut and train their clients with a "one size fits all" approach to exercise and while this may be alright for the raw beginner for the purpose of general muscle and connective tissue adaptation, they need to be able to branch out and have a vision of how to customize exercise routines that are best for your needs. In other words, what do they see possible for you. You might ask the trainer; what kind of a plan do you see for me and how will it benefit me?
Another point to consider is whether your fitness trainer can be objective about your training. Your trainer should always consider your needs during each training session. Sometimes the trainer may need to alter your exercise routine due to an injury or simply because you've had a hard day and do not have the energy you usually have. You might ask the trainer; how do you assess me each day before my session.
One of the most important things, of course, is whether you and your fitness trainer have matching personality's or simply put, you must get along with each other and you must feel comfortable with your trainer or it will be hard to work with him/her. You need to understand what your trainer is trying to do and so your trainer needs to give clear, detailed instructions. Your fitness trainer should be open to what you have to say and be able to listen to and respond to your concerns.
If you have made the decision to invest your time and money into an exercise lifestyle, and you are enlisting the help of a fitness professional, make sure you choose one who you know will give you everything you need to give you the results you want in a safe, enjoyable way.