air sealing, both during construction and especially after, is tedious, time consuming, and needs to be pursued with an obsessive zeal.
99.999% chance your builder didn't give a f@#$
you should not trust their work. move insulation out of the, pull off window/door casings, baseboards, and especially crown molding.
years before i bought my haus, a leaky roof over the kitchen forced the replacement of the entire ceiling. the drywall had an 1/8" gap where it didn't quite meet the walls, covered by cheap crown molding. when i pulled it off, the back side was black from years (decades?) of air being sucked through the gap- under quite a lot of pressure i might add due to natural stack effect. no wonder i could never keep the kitchen warm!
a blower door test is the best for finding air leaks, if you are that serious about it. google for more info about it, an awesome tool/method to seal your house!