http://www.nhsdirect.nhs.uk/articles/article.aspx?articleId=85§ionId=20102 said:
All men should examine their testicles once a month. The best place is in or after a warm bath or shower when the scrotal skin is relaxed. Using both hands, each testicle should be gently rolled between the thumb and index finger to find any lumps or irregular areas. You may want to ask your partner to help you.
You may be more at risk of testicular cancer if you have an undescended testicle, a family history of the disease or a previous testicular tumour. In these situations, it is very important to carry out self-examination at least once a month, and report any changes to your GP.
One testicle is usually bigger than the other. All men have a lumpy tubular structure on the back of the testicle called the epididymis (where sperm is stored) and a small firm tube which passes from the testicle into the groin called the vas deferens (where sperm is transported). When you get used to how your testicles feel you will recognise these parts and not mistake them for anything abnormal.