There is currently no way to see the passive stacks of heroes like:
- Renna (Passive),
- Zarus (Permanent ulti physical power. The temporary Gladiator stacks are probably not required to be visible and you can sort of gauge when the passive is active to some degree via his spear glow),
- Feng Mao (Passive),
- Sevarog (Passive).
HOTS, Dota 2 and LoL all show stack info in the ingame UI - so it shouldn't be a detriment here either.
Sev is sort of halfway there, but that's almost certainly because the models for his shoulders to represent each tier already existed since Paragon, hence it was less work to hook them up to the stacking mechanic here. You currently still can't see the actual number of passive souls, though.