New feature added to Mockito makes this even easier,
ArgumentCaptor<Person> argument = ArgumentCaptor.forClass(Person.class);
verify(mock).doSomething(argument.capture());
assertEquals("John", argument.getValue().getName());
Take a look at Mockito documentation
In case when there are more than one parameters, and capturing of only single param is desired, use other ArgumentMatchers to wrap the rest of the arguments:
verify(mock).doSomething(eq(someValue), eq(someOtherValue), argument.capture());
assertEquals("John", argument.getValue().getName());