sobpoenas

Last known address. If it's current address, I think they can give notice by posting it on your door. . .place of work . . .

Most of the common sense methods.

All of the above (except for social security numbers ... info attached to those is generally hard to access even for the police).

I did this for a short time many years ago and mostly we went on the address indicated on the subpoena and any other addresses provided to us by the involved party or parties. If we got paid enough, we would do a little hunting and searching and this was in the day before computers were everywhere. Now it would be much easier to find some people.