Christian process theologians consider Jesus to be a perfectly clear window to god. Without Jesus, then, the most effective and understandable communication is impossible. Such speculation as put forth here has no value to a Christian process theologian.
From a process perspective, these ruminations can have no value at the present time without some breakthrough in either god's or man's spiritual technology.
Polytheism poses different problems. A god may be aware of other god's existence either as the god of a localized region of space or as a god of some power. Problems arising from such a perspective include the possiblity of jealousy having given rise to wars and to truces amongst the several gods. All the gods might be reluctant to speak first for fear of reprisal from the others.
Theists, in general, may object that testing for god is an afront to god and to themselves as followers of that god. That may be so. Further, even suggesting testing may be seen as an insult.
For example, if the test were to prove a positive for a god who's designee was a Moslem I would be reluctant to follow such a god after the September 11, 2001 murders. Or, a Christian might feel Buudhists are stealing some of their thunder if a Buddhist designee passed the tests. Budhha's redeeming work under the tree; the simplicity of calling on Amita Buddha at your death as a way to leave the cycle of incarnation; the boddhisatvas working to help us poor earthlings. I know I've gotten a couple e-mails that insist that Buddhism has to be derived from Christianity.
What is a soul's function? What happens to a soul before birth and after death? How can these functions be shown to require a soul, or said another way, what about these fuctions requires a soul, and what about these functions cannot be accomplished non-soulishly?
Since a belief in god depends, at least in part, on a soul in indiviuals to go to the god upon death, a proof of souls existence and/or functionality would provide an indirect proof of god and a definite proof for one of the cornerstones of spiritual practices. It might be reasonable to conclude that the person who can reveal how to prove souls got that information from god as described by the designee.
One major problem with this approach is that technologies relevant to spirits or to control of spirits may be revealed or be derivable from such seemingly relevant information. In sum, god may wisely withhold this information.
History...or the revealing of history to be explicit may prove to be a useful street to explore. Who shot JFK? That may be a tough one to back up with evidence. Besides, it is a crime and revealing the perpetrators of this might be perceived as an underminding of freewill both by god, the populace and the criminals. The near past's most innocuous moments may be doable as a test. Maybe, but only if the moments are so inoccuous as to be nearly forgetable. Almost anything could be described and would be both irrefutable and unprovable. A volunteer populace could solve this problem if they keep minute diaries. The populace would need to be large and dispersed enough to forbid easy scrutiny by cheaters. This would only work if god approved of the populace chosen. We have a maybe.
Such a population already exists of course. People of integrity inhabit different religious orders world-wide. They may not be diarists, but that is easily solved. People who take the idea of spirits so seriously that they make spiritulaity their life's work may be disinclined to take part in such a program, not wanting to make god a circus act. On the other hand, some may be inclined to lie. A test populace from the general populace would be a possibilty but their lack of professionalism may interfere with results. Best practices of the scientific method would need to be considered in devising such a population, keeping track of their lives for verification of the designee's revealing of history.
Would a god report on the movements of, or even keep track of, individuals who practice outside of the god's preferred spiritual practices? Failure to report on practitioners outside of one religious movement might be seen to skew the results and the truthfulness of the reports. This would make the results questionable. While revealing history may seem like a method it presents too many problems to keep track of.
Bending probabilities...now this sounds like fun! If a god revealed to the designee that a certain probability or probabilites were to be bent (i.e. a miracle)this could be a sure sign. Nothing fatal or scarey...we don't want to harm people. And clearly anounced well ahead of time, a general "something big will happen" will not work even if California tumbles into the sea (that'll be the day I go back to Annandale). Specificity and timeliness, that's what's called for. Such information can be delivered about localized or general bendings.
PCs running different operating systems, configured on networks or as local machines, some with the date set correctly some not, a wide and varied test populace spread all over the world. How about if god tells the designee what earth date, use GMT to the minute if possible, not the various PC's dates and times, all PC solitaire games will turn up a well defined, and announced, hand. Now that's about as huge a proof as any atheist could want.
But if dishonsety in reporting is a problem if god is asked to provide a history lesson it is just as much a problem in reporting on bending probabilities. Some people will lie, and knowingly so, to protect their conception of god and they will justify that lie by believing that god as they conceive god cannot be bothered to compete.
And, of course, such bending of probabilities might be explainable by an appeal to advance technology. That is to say, it ain't god, it's an advanced culture, maybe from another planet, doing the bending.
So, quite possibly, no proof of god can be offered by god through a designee. The reason, in a nutshell, is simply the human desire to protect pre-existing conceptions. The only proof for god would have to come from god itself.
How this would come about is the question and the only answer I can conceive is "I don't know". I don't know...
I think personal revelation is, quite possibly, the only way of knowing that god is and what god wants, at least unless god puts in an performance that cannot be refuted.