X-ray isnt 'fine enough' to detect the macro-molecular cracks that develop with the onset of 'fatigue failure'.

You can have the part 'magnafluxed' (magnetic testing) by a lab that specialized in stainless steels mag. testing.

Dye penetrant testing is a simple method but is only a 'go-by'

There are two common 'best methods':
1. eyeball - if there is any 'dulling' or 'change' of the visible surface characteristics in comparison to non-stressed areas - condemn the part.
2. If you have completed enough travel to equate to a 'circumnavigation' (ie.: ~25,000 nMi) - condemn the part.

2a. (the 'technical answer') - anytime 300 series is loaded beyond 30,000 psi (about 30% tension on the rigging gage) it will only withstand 1 million load cycles ... then catastrophically fail via 'embrittlement' or fatigue failure. Repetitively stressed metals get 'tired'.

When in doubt ... simply replace with new. Such is many times cheaper than 'testing'.