How do I tell which resources stack and which don't?

[Ungod]

Also, how do I split the ones that don't stack?

[Demonic God]

As a general rule - resources tend to stack, whereas items, consumables, and valuables do not.

Resources are things that are gathered, or processed, using tools and creatures from the realm.

A notable exception is the memory stone - while gathered as a resource, is a consumable.

To split a resource stack, you need to use a resource splitter. They are available via the wishpoint shop and the credit shop.

[Miq]

This part is quite a mess. All in all they all stack but depending where you get them from they might end up as separate stacks.