Many schools today offer online dog training certification. A school offering certification in dog training will supply you with an XXCDT certificate, the XX being the school’s initials. The trick is to locate a reputable institution that meets your needs. Avoid schools that offer a multitude of unrelated certificate subjects. For example, a school that offers certification in gun repair, dog training and home inspection is probably not going to give you the education or the reputation you are looking for.
Look for a school that has regional or local certifications. Many good schools are able to offer financial aid of some sort. In addition, a reputable school will have job placement assistance and, once you complete your degree, will list you as a certified dog trainer on their websites. Finally, be sure that any school offering a certified dog trainer program has a practical part of the training, with live dogs. Most importantly, there are safety issues that need to be addressed and dealt with in the field before the student can start training dogs professionally. Aggression in dogs is a main reason owners look for training --- and if you don't have practical experience in addressing canine aggression, you will put yourself, the dog and others at risk.
One reputable school to look into is Animal Behavior College, a good online school. Part of their program includes assigning a mentor. This mentor will be a local graduate from ABC who will work with you for the final part of the course, which includes hands-on training. Another school, The Tom Rose School of Dog Training, is a highly recommended residency program.