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, especially if you develop repeated vomiting, confusion, collapse, severe weakness, extensive skin redness, unexplained burns, or blistering.
Tell the emergency dispatcher that radiation or radioactive material may be involved.
Move away from it and contact emergency services.
During a large public incident, use official emergency channels rather than going outside to investigate.
And for anyone, someone who:
Do not delay urgent care for serious injuries.
Arrange routine medical follow-up if you have persistent skin changes, patchy hair loss, an area that repeatedly blisters or opens, unusual bruising, or ongoing symptoms after a documented exposure.
People previously treated with radiation should also report unexpected changes in the treated area to their treatment team.
The article also says
Do not handle or transport a suspected radioactive object.
If radioactive material may be on your clothing or skin, keep away from other people when practical and follow official decontamination instructions.
Every sign above is quoted from the sofidex article on Radiation injuries. If you are not sure, err upward — it is always reasonable to be checked.