It's not possible to check if user enabled lock screen password or not as far as I know
but if you want to made your app secure (because it may include important data)
you can create a password for your own application !
I did it in a little notepad app my password page allow user to set a password with all English and Persian Characters , numbers and special Chars like !@#$ and etc.
Nokia Lumia 920
GDR3 Developer Preview
Developer Unlock : ✔
Interop Unlock : Soon...