NFPA 58 Excerpts

DOT cylinders in stationary service that are filled at the point of use and therefore are not under the jurisdiction of DOT either shall be requalified in accordance with DOT requirements or shall be visually inspected within 12 years of the date of manufacture and within every 5 years thereafter, in accordance with 5.2.3.1 through 5.2.3.3. [ROP-54] 
5.2.3.1 Any cylinder that fails one or more of the criteria in
5.2.3.3 shall not be refilled or continued in service until the condition is corrected.
5.2.3.2 Personnel shall be trained and qualified to perform inspections. Training shall be documented in accordance with Section 4.4.
5.2.3.3 Visual inspection shall be performed in accordance with the following:

1. The cylinder is checked for exposure to fire, dents, cuts, digs, gouges, and corrosion according to CGA C-6, Standard for Visual Inspection of Steel Compressed Gas Cylinders, except that paragraph 4.2.1.1(1) of that standard (which requires tare weight verification) shall not be part of the required inspection criteria.
2. The cylinder protective collar (where utilized) and the foot ring are intact and are firmly attached.
3. The cylinder is painted or coated to minimize corrosion.
4.  The cylinder pressure relief valve indicates no visible damage, corrosion of operating components, or obstructions.
5.  There is no leakage from the cylinder or its appurtenances that is detectable without the use of instruments.
6.  The cylinder is installed on a firm foundation and is not in contact with the soil.
7.  A cylinder that passes the visual examination shall be marked with the month and year of the examination followed by the letter “E” (example: 10-01E, indicating requalification in October 2001 by the external inspection method).
8. The results of the visual inspection shall be documented, and a record of the inspection shall be retained for a 5-year period.