Is there a tool or extension that can tell me where i dont have a square corner?
You can use the Protractor to measure angles and see if they are square.
Is there a specific situation where this is an issue? When you are drawing edges you can use inferencing to ensure that one edge is perpendicular to another.
You could edit the style and set edge color to By Axis which would help you identify edges which are off axis.
There are extensions that can create grids of guidelines that might help you detect out of square edges.
On the advice of the venerable Michael Brightman, my default template has a scene set up on a By Axis basis specifically to check that things that should be are on axis. Of course, it doesn’t quite do what you request which would test for perpendicularity whether a pair of lines were on axis or not.
I try to use the rectangle tool in creating most objects, even overlapping them and deleting the redundant lines to ensure 90 degrees…