If you mean, how can you find the serial number -
How to find the serial number for an iOS device even if you don't have the device - http://support.apple.com/kb/HT4061.
If you mean, you have misplaced your iPod touch -
Do you have an iCloud account, and did you set up "Find my iPhone/iPod"? You would have needed to do this prior to the loss of the device.
Free "Find my iPhone" application - http://itunes.apple.com/ca/app/find-my-iphone/id376101648?mt=8. Setting up "Find my iPhone" - http://support.apple.com/kb/PH2697. and http://support.apple.com/kb/PH2698?viewlocale=en_US. and http://www.apple.com/icloud/features/find-my-iphone.html.