But on a serious note, make a shortlist of tuners, visit their premises and speak to them. You will quickly work out based on 1st impressions who is really clued up and who is just a reseller of files (i.e can read and write software from/to your ecu and emails it off to *insert random file supplier here* )
Your remap should ideally include:
Discussion regarding fuel type used etc...
Dyno testing and/or live datalogging at the very least
Visual and OBD diagnostic inspection
The sad and worrying fact is that we are seeing more and more cars arrive at our premises with problems caused by cheap software, lack of attention and interest by the tuner, poor aftersales service from the tuner as they can only remap a car by flashing files to it and not actually understand the inner workings and science behind what they are doing.
There are plenty of clued up tuners out there with the correct tools and equipment to do a good job, you just have to fight your way through the sea of bulls**t to find them.