Typically found on the passenger side (right side) of the transmission.
You can verify if your unit is still under factory or extended coverage by entering the S/N into the Warranty Status tool on the HUB. Allison Transmission Serial Number Lookup
While a full decode often requires dealer-level access, the of your serial number can often tell you the transmission family: Transmission Series Typically found on the passenger side (right side)
The most reliable way to identify your transmission is to locate the (nameplate) attached directly to the transmission housing. Allison Transmission Serial Number Lookup