I've had it tested. Now, with some experience I can self gauge by how I feel.

I do not take exogenous testosterone but will be open to it as I age if healthy lifestyle can't keep it up.

They have blood tests for it, and a little bit of googling shows that there are home test kits too (that use saliva and are less accurate):

https://www.amazon.com/Testosterone-Level-Hormone-Balance-Te...