Test case would have detail steps of what the application 
is supposed to do. SO 
1) Functionality of application is known. 

2) In addition you can refer to Backend, i mean look into 
the Database. to gain more knowledge of the application.

According to me question is not clear,because first of all 
we dont have any documentation reference to prepare 
testcases.But in question,you mentioned we have test 
cases.How you prepared the test cases without having any 
documentation?By seeing the interface of the application?
Again there is one more question can be raised i.e at least 
without having minimum requirements,is it possible to 
preapare the testcases?

My assumption is,if you dont have single document for the 
reference to test the application,then simply we will test 
the application as per our domain experience.You can easily 
find out the expected behavior of the application as per 
the domain experince on the other applications.This is 
called adhoc testing.

Comments please....

Hi Rao, This can be happend like(For Ex) if the 
organization is outsourcing the testing phase to some third 
party and in such case they don't want to reveal their 
req's & all so they will prepare test cases and give them 
to the third party so that they will go thru the test cases 
step by step/help to understand the behavior of the 
application and perform testing

I f we doesn't have SRS and BRS , in this case . we need to 
start the testing based on knowledge of application or  
test cases from similar another project.