Yes, that' a valid test that indicates the PS pump, its only put under load when its needed (when you turn the steering wheel).

My story: I had a squeak I was chasing but it was only when the engine was warm, so I took the belt off when it was warm (ouch). Checked the water pump: it squeaked. Checked everything else: no squeak. Took the pump out: it squeaked. Checked everything else: no squeak. Put everything back together: the squeak is back. Right now I'm thinking I had multiple squeaks, what fun.

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5 speed '91 190E 2.6 320,000 mi. (new car, fast, smooth as silk six, couldn't find any more Peugeots)
5 speed '85 Peugeot 505 2.5l Turbo Diesel 266,000 mi. (old car, fast for a diesel, had 2 others)
5 speed '01 Jetta V6 (new wifes car, pretty quick)
5 speed '85 Peugeot 505 2.2l Turbo Gas 197,000 mi. (wifes car, faster, sadly gone just short of 200k )
5 speed '83 Yamaha 750 Maxim 14,000 mi. (fastest)
0 speed 4' x 8' 1800 lb Harbor Freight utility trailer (only as fast as what's pulling it)