The see the actual level of hydrogen in a tank, you go the the control meny(K-menu on PC), not the Inventory meny. In the lower right corner it should say how many Liters it's filled, same as for Energy stored in battery.
You can also see how how many LEDs are lit up, for 25, 50, 75 100% on the outside of the tank.
The only thing that goes in the Inventory of a hydrogen tank is the Hydrogen bottles for refilling