As your request is taking 5 seconds, you can easily test this by invoking two operations at the same time by using two WCF Test Client or by opening two tabs in the same WCF client.
An integration test is certainly a better choice to check this behavior.
In addition, if your want to check that the behavior is really applied to your service, you could use WCF diagnostics such as WCF counters, especially "Percent of Max Concurrent XXX".