having a trainer is great! it makes you realise your full potential and you are able to lift weight that you would never dream of by yourself!!!! i know i wouldnt lift too heavy on a chest press in fear of killing myself but knowing that they are there to help you is GREAT!!
if you go to a gym i suggest that while you are doing your normal workout that you watch some of the trainers interacting with their clients. you will get to know what type of person they are (talk, quiet, have a laugh etc), type of trainer (focus on core based exercises etc) and then you will be able to match this for what you are after. also wether their clients are getting results or not (i know a clients results arent totally reliant on the trainer but it does help)
Of course make sure you get their creditation and ask them their specialities etc but observation goes a long way
if you arent a mbr of a gym then maybe someone else might have some ideas.
either way a good trainer is def worth their $$$$!!!!