To identify software vulnerabilities that allow DoS it would be necessary to first identify all vulnerabilities, and then exploit them to study the result. Normally, this type of test is limited to identifying vulnerabilities, since the impact is usually already known.
DoS network attacks, on the other hand, tend to depend more on the infrastructure on which the service is mounted, so testing is needed to determine the impact a real attack could have. To do this, it is necessary to simulate a large amount of traffic that tries to saturate the network resources that manage the service. Specially developed tools are used in this type of test.