"There's a lot of copycats," the original shop told us when we called this afternoon to ask how to spot a fake and where they might be found. "The logo", they told us. "Their logo is different."
Indeed, the authentic Er Guang logo looks like this (used at their shops and on their Dianping page):
The fake one looks like this:
On the bottom, it says Shanghai wanghong sesame sauce wontons. As in life, the original doesn't need to shout out from the rooftops that they are special. They just are.
Alternatively, you can always refer to the trusty SmartShanghai directory. The seven real, authentic, original Er Guang locations (as of January 2019) are listed right here. If it's not on the list, it's not legit. Er Guang wansui.