Perhaps a "true" god is one that sits on an Astral fountainhead of the divine spell delivery system. They don't create the flow of divine power per se, but they can bestow it amongst those who are worthy.Which would mean, in such a setting, that faith isn't enough. What you worship also has to be a "true" god. Rather than coming from your faith, divine magic comes from the god as a reward for your faith.
In Eberron, the aloof, distant gods may be less micromanaging and direct the flow of divine power to anyone who has been put on "the list", regardless of their worthiness (ie., alignment).
There might be no other differentation between a true god and a false god other than whether that god has the position of steward/administrator in the divine spell delivery system. (This position may be well-earned or not).
Theoretically, worshippers of a demon upstart like Orcus might think they are receiving a divine boon from Orcus, but in fact, there is a true evil god who is secretly directly some divine power to the cultists for some nefarious purposes.
This is just an idea. This actually sounds more like an atheist/secular rationalized take on divine power, rather than a sacred dogma.
A cynical view: The god retains self-confidence in his place in the universe; an unworshipped or unfeared god is forgotten and fades into oblivion.Which begs the question: what do gods get from faith?
A cynical conspiracy-style view: the gods get a piece of your soul every time you cast a divine spell.
An optimistic view: Faith in your god genuinely makes the world a better place, although "better" means different things for different gods; Faith in a war god reinforces a world of glorious warfare.