I don't know which specific fluid VW uses for power-steering, but many manufacturers use automatic transmission fluid, or something closely related to it.
Unlike the engine and transmission, the power steering pump will not heat up it's fluid to extremes, the only significant heat the fluid will received is from being in the engine bay.
Therefore, power-steering fluid takes years to degrade, mostly from oxidation. Read it like any other fluid, look for a dark colour or a burnt smell.
It's easy to flush, so if you're not sure, just go for it. On most vehicles, you just pull one of the lines, let it leak out then top it up. Steer left and right to get the fluid to reach all places. If you don't want to pull a line or if it's inaccessible, steal a turkey baster from the kitchen (do NOT put it back when you're done, just make-up a believable story), and just suck the fluid out of the reservoir. Top it up, drive around for a little bit, and repeat until the liquid looks good.