A large number of assertions are available for validating received messages, some being specific for a certain type of Test Step and some being common for all Test Steps.
The assertions available for all Test Steps are: The example shown in the screenshot specifies a reqular expression checking for the string "Session ID" in the entire content of the message to be validated, note the "(?
Here we can see that the "Test Request - login" Test Step has failed, which in the Test Case Run Log at the bottom also displays details on the actual assertion failure; "took 1023 ms" means that the "SLA" assertion failed, i.e. Assertions are always displayed in a tab at the bottom of the containing Test Steps' editor window.
In the screenshot above you see the 3 assertions added to a SOAP Request Test Step and they all failed.