Might be the best tips I see on Taobao (it's eBay, but Chinese edition).

In the picture, it's trying to make sure buyers are not buying the wrong size of a light bulb, where it tough to distinguish the difference between E12 and E14, even with a regular ruler (I mean, ordinary people would be like: yeah, a little bit more than 1cm).

And this seller suggests that you can use a charger to determine the size, where you can put E12 in between the pins of a standard American 2-pin plug, while E14 can't.

That's really smart! And that's why Apple and other phone sellers should include a charge with the phone, LOL.