Well, I am not an expert in this sort of thing, but I believe they work like any ordinary transformer. That is to say the mains line is the primary, the sense xformer is the secondary. There is a ratio of current exchanged between the primary and secondary as I am sure you already know. So you can figure 1A through primary X amount of amps will flow through secondary. This info is given in the current sense xformer data sheets.
So with x current flowing through secondary rectify AC, run voltage across a resistor, and using E=IxR you know voltage. Of course you must take into account E drop through rectifier.
This may not be the best way, but it gives you a general idea. I have a feeling someone will smack me, as I probably made it over complicated.