In the absence of a Geiger counter... One of the really fun things about radiation is that -- like fiberglass dust -- we have no good way of assessing our exposure until it's much too late to change it. As a nurse and as a DIY boat-owner, I figure it's reasonable to protect myself as best I can, then hope for the best.
Tech note on seaweed/thyroid remark: the natural iodine in seaweed and miso loads up your thyroid gland's iodine receptors. This leaves no room for radioactive iodine -- carried in rainwater, for instance -- to glom onto you. It's exactly the same mechanism as the benign iodine in radiation pills. The dosage is more precise with the pills, but the taste of the seaweed treats is rather better.