Look around at General condition.
Is is bashed, scratched etc?
How is the full nipple? Worn etc?
When filling, attach the fill whip and make sure it is secure.
Hold it suspended by the nipple/gauge with 2 fingers as a v, nothing but ground below and the sky above
If in doubt have it retested before use
There will have been a certificate to accompany the test.
Assuming you don't have that the label will name the test centre. Call them with the serial no and test date and they will confirm
http://p8ntballer-forums.com/threads/hpa-bottle-test-certificate.158344/