There are a few options that I can think of.
If you just need to grab a few coordinates, you can add a graphical point, double-click properties, and click the location tab. (see picture below).
If you have a lot of corners, create a point layer, and you can add the XY to all the points automatically. Then you can use this to create labels or callouts.
If you already had the coordinated of the corners in the GIS layer you could also use the Add Label tool in the Drawing toolbar.