By doing Adhoc Testing,Explaratory Testing& Monkey testing

How do you test if you have minimal or no documentatioabout 
the product?
1. First Play with application and get the working 
knowledge about the appln from either developers are 
product engineers.
2. Start writing Positive Test cases usig the knowledge you 
gained
3. Review it thru concerned experts
4. Write all the Negative Test cases and other test cases 
if any (UI, Performance etc) 
5. Make it as your baseline Test case document for further 
testing activities.

How do you decide when you have tested enough?
Create Traceability matrix map requirements to Test cases

How do you determine what to test?
1. Tester instinct and experience vital role 
2. First Test Positive test cases so u can make sure that 
Use cases are working fine then other Test cases.

Hi,
    1. If we have minimal or no documentation, We have to 
discuss with the team lead or QA manager and also have to 
attende the review meeting with the bussiness analysts. 
Based on the review we have to analyse and write the test 
cases.
     2. This is decided by the exit criteria. This is 
prepared by the team leads and QA manager when they do the 
test plan. This exit criteria is discussed with business 
analyst and prepared.
    For ex. In some companies they set the exit criteria 
like no high level bugs, no medium bugs, 1 or 2 low level 
bugs. So when ever the testing meets this criteria, we can 
stop the testing with the approval of the test manager.

      3.Initially we have to each and every functionality. 
If the time looms then we have to do the risk analysis or 
the ad hoc testing to test the main functionalities.