Sometimes (i.e util classes) there is need to verify in tests that constructor is private. Best way in my opinion is to do it using Java reflection and below is almost drop in solution how it should be done. Only thing that needs to be replaced is the class under test

PrivateConstructorTest.java

public class PrivateConstructorTest {
    @Test
    public void utilClassConstructorMustBePrivate() throws Exception {
        // Replace  ReportBuilderUtils with class name under test
        final Constructor<ReportBuilderUtils> constructor = ReportBuilderUtils.class.getDeclaredConstructor();
        assertThat(Modifier.isPrivate(constructor.getModifiers())).isTrue();
        constructor.setAccessible(true);
        constructor.newInstance();
    }
 
}