What cuboidal cracking looks like
The timber splits both along and across the grain, leaving roughly rectangular blocks. As decay progresses, those blocks can become brittle and break away easily.
Dry old timber can also crack
Age, heat and repeated drying can produce checks in timber that are not active fungal decay. The depth of the cracks, softness, weight loss and surrounding fungal evidence help distinguish the two.
Strength loss can be greater than the surface suggests
A joist may retain its original outline while much of the internal wood has lost strength. Where the member is structural, superficial inspection is not enough if the timber feels brittle or breaks away.
Find out why the timber stayed wet
Cuboidal cracking tells you about the type of damage; it does not explain the building defect that caused it. The moisture route still needs to be traced.
Useful technical sources
These links provide background guidance. A particular building still needs its own inspection.