PetroStreet

is there any API standard for inspection of cylinders?

if not available then how we can ensure the integrity of cylinders having H2 and pressure 100kg/cm2?

Treat the cylinder like a pressure vessel and now I think you would be finding much for inspection protocols.

you mean to sa i go for penumatic or hydrotest??

You may go for AET (Acoustic Emission Testing) for so critical cylinders. See ASME Sec. V for more details on AET.
And yes, pressure testing must also be performed on these cylinders at pressure slightly higher than the normal operating conditions.
You must also consider using UT and RT controls especially at the head to shell junctions.

You are talking about DOT cylinders? Those used for temporary set-ups?

@ashfaq
these are simple cylinders used for transportation same as used for CNG or LPG

These cylinders are not really designed against ASME. See the scope of ASME Sec. VIII Div. 1 then go through API 510 (Inspection of Pressure Vessels) and see Appendix A also where code exemptions have been mentioned. Para (c) would help you in making a better judgement whether your cylinder can either be treated as a pressure vessel which may be inspected under API 510 requirements or not.

I do not know exactly to which specification / design standard the said cylinder may have been designed and fabricated but internationally, these cylinders for transportation / temporary storage are designed against different applicable design standards. In US, United States Department of Transportation (DOT) is the authority which then deals with the re-qualification, testing of these cylinders.
These cylinders are classified in certain classes like DOT-3, DOT-3A, DOT-3AA, 3B, and many others. Against their DOT specification, the minimum retest pressure (hydrostatic) and retest period are mentioned.

Now in short, it's better that you must look into some marking present on the cylinder which may provide you it's classification / nomenclature to some standard so that you may narrow down your search in order to proceed for applicable inspection. If you do not really find anything marked on it, you cannot select the applicable hydrostatic test pressure at which you may test the cylinder. However, you may go for different NDTs to check cylinder's health.