Testicular cancers are usually firm, non-tender masses. Depending on the size of the mass, it should be able to be felt on most occurances, although this is not guaranteed.
Other possible causes would be a cyst, hydrocele or varicocele. These type of lesions are more fluid, and thus may not always be felt.
The next appropriate step would be a scrotal ultrasound which would be a reasonable first step that is pretty comprehensive.
Followup with your personal physician is essential.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.
Thanks,
Kevin, M.D.