1.
Long-range Attack can kill maximum 3 units, regardless of their HP. If, let's say, 10 houses are placed close to each other, in the range of Monitor's splash damage, still only 3 will be destroyed. The same goes with weak units, as Strelets. If you aim your warship on enemy's army then, pick stronger units to kill, as the whole damage will be taken.2.
You can see whether the building has been destroyed by the Long-range Attack by noticing the burnt pieces and cinders left under the building, like on the image:
However, you must remember you destroyed it, as after a couple of second the outlook of the building will remain untouched and in this case the TP will still be shown as having full HP. If you aim your Monitor again at it, the Long-range attack will be available, but if you use it, you'll just waste a shot and you will damage the empty space, as on the image:
In fact, every Trading Post spot is available to be bombarded — of course when in range of a Monitor — regardless of the fact whether the TP is built there or not. Sometimes you may got lucky and snipe it.
In terms of Asian wonders, after destroying them you hear a characteristic sound, which may reassure that they have been taken down.
This whole point may be irrelevant if you just calculate and take into account Long-range Attack and the HP of the building you're going to attack (so the number of Monitor shots required), but sometimes you're not certain how many HP does a particular building have, whether it has any upgrades or not (like ATP or buildings HP cards) or just don't have time to calculate — the information here may then be useful.
3.
Trivia time — did you know that Spanish „Unction" card affect warships as well? Fully-upgraded (with all necessary HC cards) Spanish Mortars have 7330 Long-range Attack and can one-shot a TC (6500 HP)!
