ASTM D1414 (D 1414) covers multiple properties of O-Rings. Part of the specification includes tension testing for determining the breaking force, tensile strength, and ultimate elongation of an o-ring. The tension test is intended to be performed on a universal testing machine (tensile testing machine). If you are going to perform this test, you should read the entire specification from ASTM. This is a quick summary to decide if this test is right for you, and to point out what equipment you need to perform the test.
Here is a short video of the test: