I wouldn't call most gym 'trainers' well qualified to perform an accurate skinfold test. For one, they usually use little cheap calipers, then, they also typically do a 3-site skinfold, instead of the infinitely more accurate 7-site... and, to top it off, there is so much turnover among gym staff that you can hardly get the same person to perform the test again later, which is important because different people will measure differently.
Look for a place in your area, often called a human performance lab, that offers multiple and vaired testing like VO2 max, lactate threshhold, etc... these places will typically perform a 7-site skinfold for $10-$15 and do a solid job. Some of them offer hydrostatic weighing as well. I've done that myself, but the 7-site skinfold was virtually identical so I'll stick to that in the future.
I guess it just depends on how much accuracy matters and if you just want a general idea of progress or to really know the best estimate of your BF% and LBM.