First of all is to say, that the Screen does not auto rotate on the HD2.
If you don't have any special tool installed or added your App to a special RegKey(You should find it on Google) the HD2 will stay in Portait. So to say, WM6.5 can handle this.
Some Links:
http://www.htcphones.net/setting-screen ... n-htc-hd2/http://wmpoweruser.com/?p=10636If you have managed that detecting is more easier that you think
Looking at the Forms Paint Event may triggering to often
Using the Forms Resize Event is better
Then you can take a look at the Widht/Height of the form ^^