Dear Madam. Your son got pwnt by a scorpion bolt.

[Crispvs]

I don't know if there is any surviving evidence for how families would be contacted. I would imagine though, that as each new recruit had to provide a letter from a member of his local ordo, this could have been kept on file, which would provide information on the district he came from. It is possible that a letter could have been sent to the ordo and and that the ordo would have responsibility for tracking down the family.

As soldiers received letters and gifts from their families it is also possible that members of the dead soldier's contaberium may have known the details of his family and may have been asked to supply this information to whoever wrote to the ordo (if this was the system used).

The piece Matt quoted also suggests that families might have had to go to the fort to collect the solder's effects / money. By the time of the letter a lot of recruitment was probably local so this would not have been too hard. However, for soldiers recruited from a long way away, especially in the first century AD when this was probably more common, it might have been very difficult for families to go to the unit's base. In fact, requiring families to visit the fort to collect their rightful dues might well have been a scam on the same level of fobbing soldiers off with swamps or other marginal land on retirement rather than paying out for good land they could use.

Crispvs