Solenoid unit?
My 2001 is on its 3rd starter in the 9 years I've owned her- 1st was original and it just lumped out- clicking itself into Davy Jones' locker.
The second was a used one off Ebay that lasted 2 years and I wanted to test it when it buggered up with the symptoms you describe- it would spin but not engage the flywheel, which I think was down to the solenoid unit attached to it, as when I tested it the part that shoots forward to engage the starter wouldn't engage.
I found this video useful when testing the starter:
I didn't have a bench though, so I took the starter out and rigged up jump leads from the engine battery to do the testing on the car park. Use a decent grade wire if you don't have crocodile clipped ones and just make the necessary contacts to see what works or is faulty. Disconnect the pos/neg from battery to car first though, in case you accidentally spark the starter.
In the past, I've found a spare battery and a battery charger have also sufficed for testing starter motors, but I prefer not to fart arse about if I can help it and I don't have a garage, so roadside is my tick.
Another old trick is to smack the starter/solenoid with a piece of wood when someone is trying to turn it over, which might un-jam a sticking starter or solenoid. If this works however, I can bet you a dollar that it means the starter motor will eventually have to be removed and inspected, as it usually means it's on the way out and is telling you before you're really caught out in the middle of nowhere.