Do you mean they don't know the stallion's name, or do you mean that they owned(as an example) 4 stallions and they don't know which one is the sire to the foal?
If they do not even have a name, then there is no way to do it, however, if it is a scenario similar to what I wrote above, and if all stallions are DNA'd, and if the dam and foal are also DNA'd, they can narrow it down to the correct sire. I believe the breeder would have to pay fees for late stallion report on the mare, and for adding another mare to the stallion report, for all the DNAing etc.