You have decided you need help, which is a good decision. Then you search, and 20 websites all sound equally confident.
In most places nobody has to hold a qualification to call themselves a dog trainer. The title tells you almost nothing, so judge the method instead.
The one question that sorts most of it
Ask what happens when my dog gets it wrong. A good professional will answer plainly and in detail. Vagueness there is the answer.
Reward-based methods are what is recommended for training and behaviour change, and aversive methods carry welfare and behavioural risk. That is the line we hold, and it is not a stylistic preference.
Green flags
Reward-based, said plainly, and described the same way on the phone as on the website.
Will tell you exactly what happens when your dog gets it wrong.
Lets you observe a class before you bring your dog to one.
Small classes. 6 dogs with 1 instructor is a class, 14 is an audience.
Insured, and able to say so without being asked twice.
Happy to say a case is outside their scope, and willing to refer you on.
Sends you home with something that takes 5 minutes, not 45.
Red flags
A promised or guaranteed result, or a fixed number of sessions to a fixed outcome. Nobody can know that.
Balanced training used as a euphemism for prong or e-collar work.
Dominance, alpha or pack-leader language, or the idea that your adolescent dog is challenging you.
Refusing to say what they do to stop a behaviour, or changing the subject when you ask.
No observation allowed, and no explanation of why not.
A review of aversive training methods found them associated with worse welfare outcomes and with no training advantage over reward-based methods.
Dogs trained with aversive methods have been found to show more stress-related behaviour than dogs trained with rewards.
Punishment-based approaches are also associated with fear, avoidance and defensive aggression. That last one is why this matters more than method preference.
When you need a veterinarian, not a trainer
Some of this is not a training problem. Start with your veterinary practice, and ask for a referral to a veterinary behaviourist.
Aggression toward people or other dogs.
Any bite, however minor it looked.
Severe fear or anxiety, including panic when left alone.
Sudden unexplained behaviour change, because sudden behaviour change warrants a veterinary examination.
Anything that could be pain, because pain frequently presents as a change in behaviour rather than as obvious lameness.
What Halfgrown is, and is not
Halfgrown is written by an experienced dog owner, not by a qualified trainer or a veterinary professional. Everything factual here cites a published source, and you can read the citations.
A good local professional watching your actual dog, in your actual street, can see things we cannot. If you can afford 1 session, spend it on that.
Safety note
If your dog has bitten, has broken skin, or is frightening you or somebody in your household, get help now. Contact your veterinarian first and ask for a referral to a veterinary behaviourist. Manage the situation in the meantime rather than testing it.
Today's version, in one line: ring 1 trainer and ask what happens when my dog gets it wrong, then judge the answer.