The best way is with an ultrasound. They can measure them with relative accuracy down to .1CM. However, cost and convenience is a factor.
As far as Prot's comments that it's subjective...well yes it is. Some will just think that they are shrinking and will therefore feel like they are...even when they're not. Others actually do.
Let's look at the facts. By getting exo T (no matter where it comes from) the brain receives feedback that you have enough so it slows/stops production of LH and FSH. Without these two your testicles stop responding to them and they then consequently shrink. That is a fact. Some folks are lucky and have balls that seem resistant to this. I can speak from experience that I had, what I considered, huge balls. I am a primary sufferer so I was producing excess LH and FSH making them larger than normal....they were measured at around 2" in size. After starting TRT
they shrunk...slowly but surely. About 4 months after I started they were literally down to the size of medium grapes...less than one inch.
Now then, after starting HcG
they grew larger again. Had another measurement done (this time for a hernia and possible epididimitis) they measured at 1.5". So, HcG does cause them to grow and TRT
CAN make them shrink due to the feedback mechanism.
On a final note, yes they normally vary in size on a regular basis but remaining small is not normal.