the date will be stamped at the top. It should be something like 12*08 (that is what mine says) and that is the date it was manufactured.
If it has been tested previously, it should have a sticker, with an expiration date.
"By law, all compressed air tanks must have the month and year they were created. On the top of your air tank you'll see a series of numbers, and you'll see a series on there that have a number set that should look like 00@00 or similar. This is the 'born on' date of your air tank. Depending on when your tank was created, you'll need to get it tested periodically.
METAL Co2 tanks need to be tested every 5 years. If your tank is under 2 inches in diameter and less than 2 feet in lenght, then you do not need to get it checked. So 12 and 20 ounce tanks WILL need to be tested every 5 years."
This tank was botn in 11/05 and and will not need to be tested until the end of 2010
Sticker showing a tank that has been tested. Good for 5 years from the date on the sticker. If the sticker is removed, the born on date must be observed, and a new test will be required.