How I can indicate the maximum point on a curve

y = r^2 (0.5) - r^3;
max = NMaximize[{y, 0 < r < .5}, r]
pt = {r /. Last@max, First@max}
Plot[y, {r, 0, 0.5}, Epilog -> {PointSize[Large], Red, Point[pt]}]

ClearAll[f]
f[r_] := r^2 (0.5) - r^3

An alternative approach to mark the interior local maxima of a differentiable function f using MeshFunctions + Mesh + MeshStyle:

Plot[f[r], {r, 0, 0.5}, 
  MeshFunctions -> {If[f''[#] <= 0, f'[#], 1] &}, 
  Mesh -> {{0}},
  MeshStyle -> Directive[Red, PointSize[Large]]] 

Normal[%] /. p_Point :> {p, Text["maximum", p[[1]], {-1, -1}]}