Petra M. asked:
How do I know a dog trainer is good at training my dog?
What is a good dog trainer?
It is a trainer that has the knowledge to share in theory and practice, making the dog owner understand the principals of learning and the application of those principals.
It is suggested to test the knowledge in theorie during conversation. Do you understand what the trainer is explaining to you? Are his training philosophies based on science? Does he lay out the plan for you explaining the what to do, when and why?
Maybe it sounds like you found the trainer that will fulfil your needs of training for your dog?
Second step we recommend is to ask how many different breeds he/ she has successfully trained?
If a trainer states that he/she has trained hundreds of dogs of all different breeds, you should ask about a few videos of different dogs showing his results. Quite a few dog trainers have a demo dog that has been selected to be easy to train, plus they have been trained for years!
Just because he demonstrates a well functioning Boarder Collie, does not qualify him to train your Great Dane to the same degree within a much shorter training program.