The easiest way to test would be to use logoedit to place 2 obviously different boot logos on the player for the 2 modes, and then to power up the player a bunch of times. If you have any failures in the first ten then it's suspect. If you have multiple failures in the first twenty then I'd say it's on it's way out. You'll know if it's completely gone.
The only problem with this (or any other test for that matter) is that it doesn't really work to prove that the switch is still 100% good. Sure, if you got through 49 without any failures then you'd have to think so. But that still wouldn't preclude the switch getting it wrong 2% of the time. Furthermore, you've just eaten 1% of the switchs expected lifetime just to test.
I think that the best that you can do is to get an _inclination_ of whether the switch is good, failing or failed, and decide then whether to live with that inclination or to replace the switch. Testing further just increases the risk of failure.