The first thing to look for is the certification of the trainer. Some certifications require little effort and education. This is an unregulated industry. The gold standard certifications are ones that are NCCA-certified.
But even being certified by the gold standard certification companies does not mean that the personal trainer is qualified. A trainer may have several top certifications but if they do not keep learning and attending conferences, the certifications decrease in value. A trainer needs to keep up with the latest research in order to create the most effective and safe exercise routine for the client.
Additionally, a trainer may be highly educated but if he can not communicate properly to the client, the sessions will be ineffective. Having motivational skills and intuition are helpful as well. Knowing when to push a bit and knowing when to ease off take intuition and experience. I have oftentimes seen clients get injured in a gym due to poor judgement and poor communication and intuition on the part of the trainer.
A trainer should never jump into an exercise routine without a discussion of the client’s goals. A full health history should be taken especially for older clients and those with medical issues.
The trainer should be a good fit for the client. For example a trainer may be good for someone looking to build muscle. That same trainer may not be a good fit for a 70 year old.