We can't.
It is not possible to know about something that has absolutely no interaction with us (direct or indirect).
Such a thing would be logically consistent with something that did not exist.
This is not to say that it does not exist as part of an objective reality, but subjectively it is consistent with the non-existent.
For example, I am sure there is someone in Japan that you have no knowledge of... precisely because you have never interacted with them directly or indirectly. To you, that person is logically consistent with someone that does not exist.
It is only through interaction - direct or indirect (e.g. in this example you might know someone who knows someone who knows this person etc) - that we can garner knowledge.
And by "interaction" I don't just mean person-to-person - e.g. we know there are stars because we interact with the photons they emit that impact on our eyes etc.
If, on the other hand, he does interact with us but just doesn't let us know it is him, then we can only know him once we have identified an interaction that does not fit with the rest of the workings of the universe... i.e. an interaction that not only defies the laws of the universe as we know them currently, but defies them at an objective level as well.
If it doesn't then we can not separate "God" from nature.
Unfortunately this introduces a "God of the Gaps" - between current understanding and the objective reality.
And the only way around this is to define God as the Universe/nature, or as existence itself etc... but then we have words for that already.
And if you wish to define God beyond that (e.g. benevolent) then you're back to square one in terms of how you would know that.
Meh, just some ramblings before supper.