Cuboidal cracking is an important clue
As dry rot attacks wood, the timber can shrink and split across the grain as well as along it, leaving block-like cracking. Very dry old timber can also crack, so the surrounding evidence still matters.
Mycelium can change with conditions
Fresh growth can look pale and cotton-wool-like in humid conditions. Older growth may become thinner and greyer. Strands can remain after the active growth has changed, which makes age and activity harder to judge from appearance alone.
Spores can travel far from the fruiting body
Dry rot produces rusty brown spores that can settle on nearby surfaces and cobwebs. Spore dust is useful evidence, but spores themselves are common and only become a problem where the timber and moisture conditions allow growth.
Confirm the moisture source
A convincing diagnosis should explain why dry rot developed at that particular point in the building. If the damp source is not understood, the investigation is incomplete.
Useful technical sources
These links provide background guidance. A particular building still needs its own inspection.