Men can screen themselves by feeling for lumps in their testicles.  Men between 25 and 45 should do this every few months.  If a lump is found on one testicle, check the other one to see if the same lump is present.  If there are similar lumps on both, then it is almost certainly a normal part of the testicles.  If the lump is only on one testicle, you should consult your doctor.