When you have your watchman view towns, those with a blue and gold shield in the upper left of the picture are shielded.
So you can quickly scan this list looking for those without such shields.
Naturally they are had to find, players tend to keep their shields up, for the same reasons you do.
Even if you do find one, they may have raised shields by the time you get around to attacking them, or they may raise shields as soon as they discover they are being attacked.
Which is why players teleport beside such towns before attacking - to allow for minimal warning.
Probably your best source for the location of such towns is your order's chat.