Mostly guess work and it is considered opinion of most historians that tonnage figures were fairly inaccurate and certainly inflated if nothing other than for propaganda reasons, let alone the rewards of medals etc.
one way of positively identifying a sunk ship though was if identifying debris (life bouys, rafts, boats or name plates etc,) were found after the fact. Additionally when survivors were either picked up or questioned by the sub crews information was gleaned as to the name, cargo and tonnage would have been fairly simple to get and there are plent accounts of encounters with survivors.
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