Whew :-)
Have you ever noticed that on good days, when you are blissfully unaware of such dangers, they also sometimes ignore you ?... I've done just this twice, in good faith, very recently, to verify output polarity on two "wall-warts" (5V and not too puny output - like 2.5A). Admittedly I only lit the LED a fraction of a second each time, but I held it between my fingers and probably too close my eyes too. Given the ubiquity of such things in computers & other electronic gizmos I sort of assumed 3-to-5V should be OK... Thanks for the warrning !

Crossbow, you have convinced me. I've been shopping at lunchtime and found myself an entry-level-but-not-too-shabby multimeter, and will learn to use it.

Also your remark on batteries and rechargeables finally reminded me that my iRiver mp3 player came with a battery pack (like yours, only not USB) that I had never used and forgotten. Amazingly I managed to locate it and will repurpose it for the 770. It doesn't seem like I can take it apart without breaking it so I can't see if it has voltage regulation built-in, but if I understood you correctly I can make this my first test with the DVM: if I load it with four fresh akalines and get less than 5V output, then it does, right ?