SkiDiva is right about efficiency.
Find out what level of certification they have.
There's always a BS/MS in Exercise Science.
The NSCA (National Strength and Conditioning Assoc) has two certifications to look for, CSCS and NSCA. I work out at a top pt sports training facility and this is what all our trainers have.
It doesn't have to be expensive or one on one, either. DH and I take a functional fitness class that's very reasonable. We go twice a week. It's a really small group and they know our sports and issues well.
Keep in mind one can also get a "personal trainer certification" for $69 online.....
The truth though is you won't find a $69, 8 hour, or weekend certified trainer working in any insured gym. Liability insurance requires trainers to be certified by nationally accredited associations and there are no nationally accredited $69 or 8 hours courses. You can also get a paralegal, Medical Assistant etc certificate by similiar fashion but just the same, your only self employable. NSCA is one, ACE, NASM,IFSA are a few more of the more common ones. It is true that Crossfit certifications are only two days, however most of these people attend after a coaching internship and typically have other certifications going into it. Personally I find they know more than most other trainers so I wouldn't base the length of their program as any indication of their knowledge.
If I were choosing a trainer I would look for 2 certifications from 2 different agencies, because they are all a little different and some specialty certifications. This tells me the trainer is focused on learning, and cares about others fitness goals rather than imposing theirs onto their clients. It does get tricky in low rate commercial type gyms to find a good trainer but they do exist. Often times the good ones move onto niche gyms for higher pay and better working conditions. Training in commercial gyms is tough, the equipment is often sub-par and there is more competition for the equipment as well as space so most people get there start at one and move on.
In addition if I were in a commercial gym looking for a trainer, I would observe the trainers. Alot of these trainers are figure competitors and train everyone for hypertrophy because that is what they do, if you see a trainer doing the same thing with a 30 year old as they are with a 60 year old - not a good choice. If you are looking for conditioning and or functional training - pay attention to whether they are doing body splits - training legs one day, chest and back the next etc. If so, not a trainer for you. I would avoid a trainer who uses the following machines in their training ( I won't espouse - there is plenty of info online if you would like to know why these machines are problematic ) the smith machine, leg extension, seated hip machines, and the abdominal rotation, seated ab crunch or back extension machine.
Listen to their clients feedback - if they come to their session everyday with something that hurts and the trainer disregards it, might be a red flag. Pay attention to what other clients are saying, if they all hurt then you will too. Find the trainer whose clients come in excited because they either feel better or accomplished something over the weekend they weren't able to do two weeks ago. These trainers come in all ages so don't let that deter you, as long as they are passionate and determined they will be good. A trainer who records a session is always a good sign too.