Bandobras Took
Emperor
. . . are defending an opposing city? I just picked up Complete off of Steam and my memory's a bit vague. 

This works of course only if you have more bombard units than the enemy has defenders.
So there's really no practical way before the Industrial Era?
Properly used artillery can be worth a lot. Trebuchets are good for attacking cities.
AI will defend its capital and cities with wonders with more units than regular settlements. That helps for guessing the amount of defenders.
Oh I quite agree, it's really annoying not being able to right click on a city and get a drop-down of units the same as you would if they were in the open field during warfare. I guess they really wanted to make the Spy function be more useful. As if ancient age people didn't have spies, lol.