It's just a bypass valve - nothing fancy. There's an adjustment screw on them too.
When the engine is cold (high idle condition) the ISV should be open slightly (you can see light through it)
(Picture stolen from internetz)
When the engine is warm (normal idle condition) the ISV should be closed.
The ISV closes as it heats up - it has an internal heater and on the Polo is also mounted on the water pipe behind the block, so will have some conducted and radiated heat.
For the bench test just use the heating element and see if it closes up.
If you need a new one, I think some of the earlier K-Jet Golfs ran a similar valve.
Hope this helps
Pete