You cannot tell: One of the quarters in the photo is a regular quarter, the other is a 2-headed quarter.
The reason you can't tell the difference, is that the two-headed quarter is made from real quarters! It's the same thickness as a regular quarter, and the edge looks the same too! It's nearly impossible to tell. This is a handy item when you need to ensure the toss goes your way.
![View Larger Image](javascript:zoom("http://static.zoovy.com/img/therestlessmouse/-/M/ma1215.jpg")){kind=link}