Is my OMP working?
 

How do I know if it's working??

Silly question, I know...

Depends on what series car you have. If this is an S5, you would know when it's not working due to the ECU throwing a code or the ECU either getting fried in the process of the OMP going out. On an S4, they are run mechanically and I would think an easy way to tell whether it works is to take a look at the lines.

On an S4, the FSM states you should disconnect two of the lines directly on the OMP and replace with a "suitable" line that goes to a bucket or measuring device. Then pull the rod on the OMP so it allows oil to flow through. You should see oil spurting out. The same principle would apply to an S5, only you would have to just increase the RPM via the throttle since it's controlled electronically.

Cleanest way I would do it would be to just stare at the lines while the engine runs...