Need to browse the values in all vertices and all quadrature points

Hello,
I ask for help from the community to post-process my results (I am a new user of fenics). I Need to browse the value of a stress tensor and a vector in all vertices and in all quadrature points. Trough this browsing I want to compare these values to a fixed threshold. If the latter is higher to the threshold, I change the value.
Thank you for your help.