Lots and lots of methods of testing. So much depends on data you haven't told us (i.e. ANYTHING useful)
You do a fitness for purpose assessment or, I would suggest, get someone else to do it who knows what they are doing.
If you want some reading try ASME FFF-1 or ASME B31G.
This is the intro to FFS-1
Fitness-For-Service ( FFS ) assessments are quantitative engineering evaluations that are performed to
demonstrate the structural integrity of an in-service component that may contain a flaw or damage, or that may
be operating under a specific condition that might cause a failure. This Standard provides guidance for
conducting FFS assessments using methodologies specifically prepared for pressurized equipment. The
guidelines provided in this Standard can be used to make run-repair-replace decisions to help determine if
components in pressurized equipment containing flaws that have been identified by inspection can continue to
operate safely for some period of time. These FFS assessments are currently recognized and referenced by
the API Codes and Standards (510, 570, & 653), and by NB-23 as suitable means for evaluating the structural
integrity of pressure vessels, piping systems and storage tanks where inspection has revealed degradation
and flaws in the equipment.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.