Snowball Items
 

I was just wondering whether the bonuses of these items at 20 stacks work properly...

in the last game i was playing warwick and bought myself a Sword of Occult and the Leviathan Armor. After ganking for a while i got 20 stacks on both items but i didnt noticed any difference. It didnt seemed as if i was moving 15% faster.
The 15% damage reduction was too hard to notice because the enemy champions werent able to do **** against me anyway.

Meijals Soulstealer is probably working, but does anyone know about the other items?

There's a quantifiable way to determine these values.

Look at your movespeed with 20 stacks, see if it is higher than just your base + boots (use someone with nothing that boosts his move speed and no masteries/runes/items that boost it).

Testing the DR is a little harder, it'd probably require a practice game.