As a wise person once said there is no substitute to hard work.
The only way one can effectively test is by ensuring Sufficient Test Coverage and Testing Effort Automated or otherwise.
The challenge could be lack of documentation, but the advantage could be close communication between team members thereby resulting in greater clarity of thought and understanding of the system.
Each Time Code is changed Regression Testing is done. Test Coverage is ensured by having Automated Scripts and the right mix/combination of People executing the Test Cases.
Exploratory Testing may also be encouraged. Exploratory Tests are not pre-designed or pre-defined. The Tests are designed and executed immediately. Similarly ad hoc testing may also be encouraged. Ad-hoc testing is done based on the tester’s experience and skills.
While automated Test Cases will ensure that the Test Cases scripted are executed as defined, the team may not have enough time to design and script all the test cases.