Hello from JustAnswer. This is Dr. Love and I would be glad to assist you today.
If there is a painless hard spot on the testicle, this is something for which you should be seen. There are several possible causes of a painless hard spot, but the greatest concern would be a cancer of the testicle. There are several benign growths that can occur, particularly of the tube that lays on top of the testicle, such as a hydrocele or a spermatocele. Both the benign and cancerous growths are usually painless, so that does not help to differentiate.
If there is diffuse hardness of the testicle, that could be due to inflammation or infection of the testicle, but a hard spot usually indicates one of the growths above.
It is difficult to differentiate these possibilities by description. Your doctor can get some idea of the cause by the findings on exam, but it frequently requires an imaging test, such as an ultrasound, to fully determine the cause of a hard spot on the testicle.
If I can provide any additional information, please let me know.