It will tell you the siege bonus on the unit or item. So like a Gate Cleaver will show you an icon on the item that it gives you +50 Troops for Sieging when that unit sieges.

Or Siege Engineers it will display like an Icon that has an arm with a hammer in it and (+10) beside it. There will also be one for supplies, and other things. I forget the thread, but me and Cherrypie goaded him into doing it on a work night.