Well you cant, you hope the chosen supplier will be good at what they say they can do but the best way is to search on the internet to start off with and look for any bad press the company may have. Check the suppliers website during the quoting stage and see how good the English is and what processes they offer and if it matches the process you are looking for.
Your elimination method during the quoting phase should have answered this trust question for you so give the supplier the benefit of your doubt and see how you get on.
To choose the right rapid prototype company you need to do your homework. I would suggest that you find at least 3-5 different companies. This method is applied in the engineering industry before orders are placed.
There reason to get a number of prices is firstly to show you the difference in cost over two countries, secondly you will see if one company is too cheap (warning signs) and third to play them off each other for the best price.