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I bought my car at 64k. I had the same problem wasn't sure either but these plugs are good to 100k I did mine at 94k glad I did em slightly early. I don't think you can pull a certain one out and check.
My mechanic was a certified Toyota mechanic, I tired to change my spark plugs at 60 k and he told me that my spark plugs are good til AT LEAST 90k. IM at 70 K right now and will replace them at 90 k or if I start experiencing any misfire.
Not to question his knowledge, but no Toyota in North America uses the same plugs that the IS does, so while Toyota vehicles that use Iridium plus are good to 120,000 miles, Toyota/Lexus recommends replacing the ones in the IS at 60,000 miles (they are not the same Iridium Long-Life plugs). I see no reason to try to push the plugs 50% past their recommended life cycle.