Making sure that toilet tissue dissolves in water is one of the most important tests that can be run. This is especially true if the toilet tissue you're using is going to wind up in a septic tank on your property or in your vehicle (for those who drive RVs). You can perform a simple test at home to see just how well a particular brand of toilet tissue dissolves, which will save you a lot of time and frustration later on down the line.
Fill the jar with water, then tear a single square of the toilet tissue off the roll. Place that toilet tissue into the jar and let it sink in the water.
Screw the cap onto the jar. Make sure there's some air in the jar to allow the water inside to flow.
Shake the jar vigorously. As you shake the jar, the water will slosh violently inside. That motion will reduce the toilet tissue to tiny pieces which will eventually dissolve in the water.