As a QA our main focus should be on testing server side validation rather than testing client side validation (though it is also imp.). For that we can try manipulating the POST parameters using some addons/tools. One of which is Tamper data(manipulates POST parameters only) or Fiddler tool can also be used it can manipulate GET and POST parameters), there are many others too.
Lets take an example explaining why it's important.
Think of dropdown containing values from 1 to 10 and is being used in some calculation. From users perspective s/he may choose any value. Now if a QA manipulates the POST parameters after the selection is done by user and in between the value is changed before the request reaches to server. If the server side validation is not done then the changed value can affect the calculations or the application.We should take care of it.
Be Skeptic, Test More!