Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Seek emergency help after a known or suspected major radiation incident if you have repeated vomiting, confusion, fainting, breathing difficulty, severe weakness, extensive burns or blistering, uncontrolled bleeding, or a serious injury.
Follow emergency-service and public health directions if radioactive contamination may be present.
If possible, avoid spreading loose material, but do not delay lifesaving care to wash or change clothes.
Use the specific emergency instructions provided by your treatment team.
During or after cancer treatment, seek urgent advice for fever, shaking chills, new breathing problems, inability to keep fluids down, severe diarrhea, unusual bleeding, or rapidly worsening pain.
Arrange routine medical follow-up for a persistent skin change, a wound that is not healing, new swelling, ongoing bowel or bladder symptoms, vision changes, or other problems in an area that received radiation.
New symptoms years later do not automatically mean radiation damage, but they should be assessed in the context of your exposure history.
Every sign above is quoted from the sofidex article on Radiation effects. If you are not sure, err upward — it is always reasonable to be checked.