Collin: There is next to nothing different between the two fifth generation Mustangs.
The only differences were specific options and equipment which became available in 2006:
The new model year brings a handful of changes for the Mustang. V6 models can now get the Pony Package, which includes an upgraded suspension with larger stabilizer bars, 17-inch wheels, ABS, traction control, foglamps and unique trim. The GT gains two additional wheel options, both 18 inches in size, complemented by unique suspension tuning. An additional darker aluminum interior trim upgrade package is now available.
You look at the VIN number!
If the seventeenth character (digit) is 5 then it is a 2005
If the seventeenth character (digit) is 6 then it is a 2006
Car spotting the difference by appearance is impossible, there were no trim/style changes.
The V6 model got the Pony Package
On the GT, the manual cars had 3.55 gears on the 2005 models on the 2016 models the manual GT came with 3.31 gears standard, with 3.55 gears being optional.
There is a tag in the driver side door and one under the hood
you check the VIN#, otherwise they will be basically the exact same car