Most airports have some sort of desk just behind the transit immigration where there's either a customs officer waiting or where you can call them. In some cases, you exit immigration with arriving passenger and go through either the green or red channel (usually at smaller airports with no proper transit immigration).

How the respective airports and authorities handle controls is irrelevant as you have the duty to declare goods that need to be declared. Failure to do so may result in fines. Ignoring the rules or implying that these rules aren't enforced is thus extremely bad advice.