Put a strong magnet to them, true stainless is only mildly magnetic/ferrous.

If you've heated the knives, or the factory did too extensively the chromium and carbon could have been "displaced" without proper tempering afterward this could cause the knives to not be able to take a good edge.

Edit: This is why stainless is so much harder to weld properly than other metals. I think it's just beyond 770F the S "Standard" stainless characteristics of anti-oxidation start to fade away.

And than you have S and L designations, L being the far more favorable for welding and brazing, S being Standard, not so much.