It is a task so monumentally difficult that only a select few have ever found a virus on their iPad, and they had to first jailbreak their device and then cruz the dark and sleazy streets, propositioning questionable apps and developers.
There are no iPad viruses for a stock iPad. For jailbroken iPads the Cydia store is just as safe as the App Store. Venture beyond this with the same common sense and caution you would use when downloading computer programs from the internet.
That said; you can not scan for an iPad virus. The few security/anti-virus apps out there are mostly educational.