When buying a suit, what needs to be checked for shatnez if at all? Does the pants and jacket need checking?
In modern tailoring, finding shatnez in suits is extremely rare. Nevertheless it has become widespread custom that wool suits be checked for shatnez. When taken to be checked, both the pants and jacket are checked, as both have places that are more susceptible to shatnez and the higher risk [relatively] areas are checked in both.
Igros Moshe Y:D 1:72 at the end of the tshuva mentions the fact that with a vast majority of non shatnez, the checking is a stringency, to ideally clarify with certainty when possible.