Just as in real world, there isn't really a way (and there are numerous real incidents of ships that were left for dead but managed to survive and make it to port). The only real way to make sure is to get it down to a state where it is listed as sunk.
One tip, though: regardless of how damaged a ship is, if you leave the area before it sinks it will not be counted. So the ship you hit must not have been far.
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There are only forty people in the world and five of them are hamburgers.
-Don Van Vliet (aka Captain Beefheart)
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