lX = RandomVariate[UniformDistribution[{-0.4, 0.4}], {1000}];
lY = RandomVariate[UniformDistribution[{-0.2, 0.2}], {1000}];
lZ = RandomVariate[UniformDistribution[{-0.2, 0.2}], {1000}];
lPoints = Thread[{lX,lY,lZ}];

g = Graphics3D[{Opacity[.09], EdgeForm[Opacity[.1]], Polygon[#,
        VertexColors -> Table[Hue[RandomReal[]], {Length[#]}]] & /@
        Cases[Normal[ExampleData[{"Geometry3D", "Cow"}]],
        Polygon[x_, ___] :> x, {0, Infinity}]}, Lighting -> "Neutral",
        ImageSize -> 400, Axes -> True];

Show[g, ListPointPlot3D[lPoints]]

side[{P_, Q_, R_, ___}, X_] := Det@Differences[{X, P, Q, R}];
insideQ[polyhedron_, point_] := And @@ Positive[side[#, point] & /@ polyhedron];
pg = Graphics3D[Point[lPoints,
         VertexColors -> (insideQ[polygonCoords, #] & /@
         lPoints /. {True -> Red,
         False -> Directive[Opacity[0.5], Blue]})], Axes -> True];

reg = BoundaryDiscretizeGraphics[g]