Hi Partha,
Can you please check whether any program dump logged in ST22 after workflow execution?
If no then you can try below mentioned approach to test WF Activity step execution:
You can add some new parameter as export parameter in BO custom method. Further, set this new parameter with some value inside the method and return it from method to workflow task.
After workflow execution, if given parameter is getting returned then it will show you that activity step executed successfully.
Regards,
Sudeesh Soni