the def I buy is in a cardboard box. Now to use it and and return it with no obvious indication of use would require
1) opening the perforations to get to the top of the jug and then finding a way to seal the perforations back up
2) removing the seal on the jug and replacing it sealed back down.
3) not using the enclosed DEF nozzle that is sealed in plastic.
Doesn't sound very doable to me.
What I do check is
1) the date code on the box
2) useing a refractometer on a sample of the fluid to show the correct DEF concentration