Well it's not too hard. You need a flash light. Look under the hood and find the power steering reservoir, trace the lines to the pump, trace the pump lines to the rack. You'll see leakage if there are wet spots at the fittings. All of the components are on the driverís side of the engine compartment.
Now if the rack itself is leaking you can tell by squeezing the accordion boots ends and if they are squishy there is fluid in them. I've not heard of E90 racks having this problem though. To get a better look at the rack, you need to pull the engine belly pan, which requires lifting the car and removing about 16 small bolts.