The definition of type radical is that expr is of type `^` and op(2, expr) (the exponent) is of type fraction.

$\mathrm{type}\left(2^{\frac{1}{2}}\,\mathrm{radical}\right)$

$\mathrm{true}$

$\mathrm{type}\left(y^{\frac{2}{5}}\,\mathrm{radical}\right)$

$\mathrm{true}$

$\mathrm{type}\left(y^3\,\mathrm{radical}\right)$

$\mathrm{false}$

$\mathrm{type}\left(\mathrm{sqrt}\left(x^2+y^2\right)\,\mathrm{radical}\right)$

$\mathrm{true}$