Fuel fleas are typically rich in uranium-238 and contain an abundance of insoluble fission products.
Due to their high beta activity, they can be detected by a Geiger counter.
Their gamma output can allow analysis of their isotope composition (and therefore their age and origin) by a gamma-ray spectrometer.
[1] An exception would be if the flea was embedded in a particularly vulnerable organ such as the cornea of the eye or inhaled into the lungs.
In water-cooled reactors, this can be due to the reaction of the zirconium alloy cladding with the cooling water, which produces hydrogen.