Never considered testing.. It says on the bottles "approved by all OEM truck manufacturers" . If it's approved is there really a need to test? . What if it doesn't test to spec, what's it doing? Or not doing?
It degrades with heat and time. It has a great shelf life at 70 degrees F, but at 90 it degrades exponentially. This is why you should (in the southwest) run just a partially full DEF tank, filling it to the top only when loading the vehicle. However if you are running it hard or every day then of course you are using it fast enough to keep it fresh.
In my case, I don't run my truck hard, and do not tow much between RV trips. My DEF last a long time. When I tow, I top off the DEF tank, and add as required.
As far as testing is concerned, I buy from the cheapest supplier. You can never tell when a pallet of DEF has been stored outside. What got me started testing is when my DEF sensor went out at nearly the same time I added DEF. I could not tell if it was bad DEF or something wrong with the truck. Turns out it was the DEF sensor. So for me, it is nice to be able to state that I tested the DEF, it must be the truck.
