1) Detecting the boundary of the surface, object etc.
2) Finding the normal on the boundary.
Sinc you have solution from PDE, you will have exact information of boundary conditions using boundary function. Once you have boundary available apply evaluateCGradientwhich gives flux of the PDE. Flux actually flows as outward normal vector to the surface and thus this function will help you get the normal vector.
There are various features available in this function. You can take a look at this link: