I've had tanks tested in the past, there are two types of testing required both of these tests occur at different times through the life of the tank.

1. Hyrdo test which is done every 5 years, (varies from State to State). This is a pressure, seal and valve test.

2. Certification test done every 10 years, I do not know what is involved but it has been recommended to seek a replacement as the costs involved can be beneficial to replacing the tank rather than testing.
I assume this depends how the tank is installed and hours required to replace and fit.