The only I have been able to have Twine tell me it is on external power was to remove the batteries and it will say 'external power'. Or if the batteries drop below the 3.0x volts, you can determine by removing the USB power and see what the voltage says, then plug it back in and see it rise back to the 3.0x volts.
I am also running on external USB power and I keep getting low-battery notifications. Can't tell at this point if my Twine is really running off of USB power or not.
I also get Low battery notifications on USB power too. I know the USB power works. Why would twine use battery power when USB is connected. Seems like it should be smart enough to know. Why bother using USB power if I have to change the batteries all the time?
Try resaving your rules to get a firmware update, we fixed the external power readings in 1.3.0. Even if your Twine shows the battery levels, it should still be using external power if it is plugged in.
Any microUSB cable that is certified for charging will work.
We regularly use old cell phone chargers, you shouldn't have any problems with old iPhone style USB charger (just make sure it's USB to microUSB, not miniUSB).
Answers
Can it be an iPhone style USB charger with a USB to mini cable?
Certified by whom?