How to Find Bapi for Particular Transaction in SAP

How to find function module or Bapi for particular transaction in sap?
If you mean that you need to know what BAPI's a particular tranx uses, which I can only assume that's what you mean, then you should access the code behind the transaction and search for 'CALL'. That normally is the standard method that think that most people use. 
Suppose you want to find the bapi for getting employee data, you usually use transaction ZHR820 for this. 
1. Find out the package of the transaction. 
run ZHR820 go to system --> status. 
Double click on transaction 
Package is ZHR
Open this package in SE80 
Open business engineering-->Business object types 
Find the BO which sounds the most appropriate 
I would suggest ZHRPROB
Double click. 
Open methods. 
Find the released method with from data or something similar in the name 
, Createfromdat2 
Position the cursor in it and click the program button 
Scroll down to find the bapi used in this method 
With this way you can also find out programs and FM's 
2. Start ZHR820 go to system-->status 
Double click transaction ZHR820
Double click on package 
Read the application component. (this is SD-SLS Sales) 
Then open the transaction BAPI 
Sales and distribution-->Sales-->sales order 
createfromdat2

- Vinod Kotiya