From the net:

During World War 1, American volunteers from all parts of the country filled the newly formed flying squadrons. Some were wealthy scions attending colleges such as Yale and Harvard who quit in mid-term to join the war. In one squadron, a wealthy lieutenant ordered medallions struck in solid bronze and presented them to his unit. One young pilot placed the medallion in a small leather pouch that he wore about his neck.

Shortly after acquiring the medallions, the pilots’ aircraft was severely damaged by ground fire. He was forced to land behind enemy lines and was immediately captured by a German patrol. In order to discourage his escape, the Germans took all of his personal identification except for the small leather pouch around his neck. In the meantime, he was taken to a small French town near the front. Taking advantage of a bombardment that night, he escaped. However, he was without personal identification.

He succeeded in avoiding German patrols by donning civilian attire and reached the front lines. With great difficulty, he crossed no-man's land. Eventually, he stumbled onto a French outpost. Unfortunately, saboteurs had plagued the French in the sector. They sometimes masqueraded as civilians and wore civilian clothes. Not recognizing the young pilot's American accent, the French thought him to be a saboteur and made ready to execute him. He had no identification to prove his allegiance, but he did have his leather pouch containing the medallion. He showed the medallion to his would-be executioners and one of his French captors recognized the squadron insignia on the medallion. They delayed his execution long enough for him to confirm his identity. Instead of shooting him they gave him a bottle of wine.

Back at his squadron, it became tradition to ensure that all members carried their medallion or coin at all times. This was accomplished through challenge in the following manner - a challenger would ask to see the medallion. If the challenged could not produce a medallion, they were required to buy a drink of choice for the member who challenged them. If the challenged member produced a medallion, then the challenging member was required to pay for the drink. This tradition continued on throughout the war and for many years after the war while surviving members of the squadron were still alive.

"Coin Check" Rules

1. A “Coin Check” consists of a challenge and response. A challenge is initiated by either holding your coin in the air or slamming it on a table or floor and yelling “Coin Check!”

2. Individual(s) challenged must respond by showing their Coin with their own unit’s logo to the challenger within 10 seconds.

3. Anyone challenged who doesn’t show their Coin must buy a round of drinks for all challenged, including the challenger.

4. Coin Checks are permitted anywhere and anytime.

5. If everyone being challenged produces their Coin, the challenger must buy a round of drinks for all challenged.

6. If you accidently drop your Coin and it makes an audible sound on impact, then you “accidentally” initiated a Coin Check.

7. There are no exceptions to the rules. They apply to clothed or un-clothed. One step and an arms reach are allowed.

8. A Coin is a Coin. They are not belt buckles, key chains or necklaces. Coins worn in a holder around the neck are valid.

ALWAYS CARRY YOUR COIN AS A REMINDER OF YOUR COMMITMENT AND CONNECTION TO SOMETHING BIGGER IN YOUR DAILY LIFE.