There is no way to non-distructively test a stainless weld in a galvanized chain. Even x-ray has some risk.
Galvanized chain will stretch if the maximum safe working load is exceeded - and you will easily see the stretch when you recover your chain.
Stainless work hardens and snaps under these conditions. You will see this when you recover the stub of the chain and leave the joiner behind.
Joined chain and stainless chain are fine for day use is settled weather
. If you can reasonably expect you might be exposed to ultimate conditions, at least you will always know where your ground tackle will fail.