The life span of cylinders may be 10 years or 15 years, check for your country’s standard. To determine the expiration date of your LPG cylinder, use the following approved formula:
Expiration date = Year of manufacture + Life span of LPG cylinder.

LPG gas cylinder testing must be done every 10 or 15 years by a licensed test station. LPG gas cylinder testing procedure may also include some refurbishment and/or coating with anti-rust paint. If the Test date was 2015/08 and the requalification date was 2020/08, then the testing interval is 5 years.

The next time to test your cylinder will then be can be determined as follow:
New test month: Requalification date (2020/08) + Testing Interval (5 years) = 2025/08.

The only test that end-users can perform is to test for valves or pipes leaks by using soapy water to check for leaks.
For proper testing, you have to take your cylinder to a qualified testing station where the cylinder is subjected to a high-pressure test.

Even if your cylinder is new it is possible that during transportation, physical damage might have affected its structural integrity during loading and unloading of the cylinder. If you are not sure if your cylinder is still safe to use or it has been a long time when it was used last or perhaps you can’t see any visible date marking on the cylinder, take it to a qualified testing station for qualification test.