There are various tests that may be involved for diagnosing a disease. Now, no matter how inconvinient they may seem to be, they are an absolute must and so they must be carried out most definately. I am listing out some of the tests that my mom went through, although I am sure that they may be many many more.
1.) Blood Tests : There are many kinds of blood tests that may be required to be performed before a suitable course of action can be decided apon.
2.) Imaging : This as the name suggests, is done to get a better idea of what is going on inside one's body. Well, to basically see, if the internal organs are in the correct shape or not, if at all there may be some kind of growth near any of the organs that may be attributing to the disease. Again, there are many tests are far as imaging goes, ranging from a simple X-Ray, to a Mamogram , to a PET Scan.
3.) Apart from these tests, there may also be some forms of biopsies that may be involved to effectively test the cells inside the body basically to check malignancy of the infected cells. An FNAC is the most common form of a biopsy used to test malignant cells.
Here is a link, which explains all these tests in great detail
http://www.cancure.org/tests_to_detect_cancer.htm
bravenet.com