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 creating a sales order, you usually use transaction VA01 for this. 

1. Find out the package of the transaction. 
Start Va01 go to system --> status. 
Double click on transaction 
Package is VA 
Open this package in SE80 
Open business engineering-->Business object types 
Find the BO which sounds the most appropriate 
I would suggest BUS2032 Sales Order 
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 va01 go to system-->status 
Double click transaction VA01 
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 

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