You might read about the prebate mechanism on the THOMAS website - http://.thomas.loc.gov - and enter "hr25" (without the quotes) after selecting the choice for "bill number".
It is actually pretty strightforward and can be handled by the SSA much as they now do for SS payments (or wire transfers). It is a highly computerized operation.
I don't recall any who support the FairTax as claiming that the prebate (or anything else) would be without fraud. Whenever you have people, you have the potential for fraud. I personally think the opportunity for fraud is fairly low however.
The prebate is based upon family size ONLY and everyone may obtain it (unless you choose not to for some reason). It is your money to spend and you choose what (or whether) you spend it - and when. Seems to me that is fair.
But please read the bill (it's in English, not legaleese) and the prebate is called the Family Consumption Allowance.
The vast majority of transactions under the NRST would require the collusion of two people to effect a fraud while the current system let's you cheat alone. The fraud losses under a NRST probably would not approach the gains from the taxing of the underground cash economy. In fact, if you assume fraud losses under the NRST would stay about the same (I think they will decrease) the additional underground tax base would be a huge net increase in revenue. Add in a few tens of millions of foreign tourists and...well?
My apologies for the broken link ... please try http://thomas.loc.gov
The "loc" stands for "library of Congress" BTW - an official site.