When you are placing an order with an online broker you should already know what exchange or exchanges that stock trades on.
For example if you look up under Yahoo Finance:
- BAC - Bank of America - it shows it is traded on the NYSE
- AAPL - Apple Inc. - it shows it is traded on the Nasdaq
- CBA.AX - Commonwealth Bank of Australia - it shows it is traded on the ASX
- NWS.AX - News Corp - it shows it is traded on the ASX
- NWS - News Corp - it shows it is traded on the Nasdaq
Notice how News Corp is traded both on the ASX and the Nasdaq. The difference is the shares traded on the ASX have the extension .AX, that is how you know the difference between them.
When you are putting orders in with your online broker you will need to select the exchange you wish your order to go to (if your broker allows trading on multiple exchanges). So you should always know which exchange your order goes to.