Any test to identify the current body condition after radiation

Since your sister had a mastectomy, obviously the emphasis on looking for BC there will be reduced But she will still have mammograms on the remaining breast and perhaps ultrasounds or other types of screening on the MX side. This is not to prove she is completely recovered from cancer but to catch any new activity early.

Unfortunately there is no "recovered completely" from breast cancer. Many women have preliminary work ups including cat scans and pet scans to be sure there has been no spread of the BC to other areas, like bone, lung, liver or brain. Some women who are at high  risk for metastasis have followup cat scans periodically. Many doctors do "tumor markers" looking for suspicious proteins in the blood. Routine breast exams are done and so is routine lab work. Most women are seen in follow up, at first, every three months, and with the passage  of time, less often. Any time something worrisome, different or mysterious occurs we are encouraged to check with our doctor.

Some types of breast cancer have higher recurrence risk within the first couple of years ; other types of breast cancer can show back up 15 or 20 years later. The good news is that most people do not have a recurrence. But there is no way of knowing for sure what your future holds. No magic test that rules in or out recurrence.  There are clues, like grade, and tests like the Oncotype that make statistical predictions. But as is quoted on this site often, your risk is either 0 or 100%, you either do or don't. 

Our news media and medical reporting seems to have perpetrated the idea that breast cancer is curable. The truth is that it is treatable and the treatment may or may not cure the patient but we have to remain vigilant for the rest of our lives.

Best wishes to your sister and she is fortunate to have a loving and concerned sister.