When laws, procedures, or acts directly violate the constitution, they are unconstitutional.
All others are considered constitutional unless the country in question has a mechanism for challenging laws as unconstitutional.
Depending on the type of legal system, a statute may be declared unconstitutional by any court or only by special constitutional courts with authority to rule on the validity of a statute.
That can occur either because the country has no codified constitution that laws must conform to like in the United Kingdom and New Zealand or because the constitution is codified, but no court has the authority to strike down laws on the basis of it like in the Netherlands and Switzerland.
In many jurisdictions, the supreme court or constitutional court is the final legal arbiter that renders an opinion on whether a law or an action of a government official is constitutional.