Personal trainers are about as useful as chocolate fire guards. Find a proper "coach" if you need one.
Unfortunate, but oftentimes far too accurate with the multitude of trainers out there who took a quick course and decided they wanted a new career path when they're still miles away from having any idea how to even get themselves to the physique they would like
The comment about interviewing multiple people who have sound coaching experience and have produced top-placers in their respective competitions is the way to go. Sometimes, you'll find that one person can't do it all, and you may need to have a coach for training and someone to help with diet (in the event you find one who is awesome in one area but may lack in another). That's the one issue we have at our faciity - Kirk (our sole trainer) is freaking INCREDIBLE at getting people stronger and to their strength/mass gaining goals at an amazing pace, but he admits he's really bad at helping people with difficult pre-contest diets if they need absolute hand-tailored precision for getting ripped. In which case, he has someone to refer clients to that can help them get in perfect shape to step on stage, as they work together closely and keep everything in line the way it needs to be. If you ever find yourself working with a great coach or a great diet expert, that person should always be very open to working hand-in-hand with anyone else who may be helping in areas that they aren't as well-versed in.
Don't be afraid if it isn't easy to find one person who is perfect in all facets - you may get lucky and find one who can do great on both diet and training, but sometimes, if you want the absolute best, you may want one who handles each, depending on level of commitment to what you're after as well as finances. Though, for most people, simply finding someone who is very good at both will be MUCH better than just trying to go it solo if you need some help with goals that include contest preparation!