How to know if Android Device Manager's erase phone worked on a stolen device? Phone was stolen, and when I went to wipe it in the Android Device Manager the phone appears off line for more than 5 days. 

Today when i checked, the phone was no longer listed in my device manager, does this mean it was wiped, how can i confirm this ?
There is no confirmation provided that the device had been wiped as you can read in Google Support (there are are earlier references and this is the latest I could find - last month)

So you can't - the fact that device is wiped also means that your device is not associated with your account and therefore you cannot see it listed in your devices. Also as stated here


  Erase
  Permanently deletes all data on your device (but might not delete SD cards). After you erase, Find My Device won't work on the device


(Emphasis Supplied)

Not appearing in the devices list cannot be a confirmation by itself that the device is wiped ; since Google didn't cite this as proof, but yes, most likely it is wiped

Unless you installed a third party device manager app, which provides such confirmation , there is no way to get a confirmation