the only way to be sure

I was intending to do the rotate-two-chains-regularly thing with this spring's cassette in order to try and eke it out but it's done a few hundred more miles than I was intending before I got time to swap the chain over and install the accompanying new chainring to replace the finally-starting-to-look-a-bit-worn twenty-off-year-old steel chainring from my old bike, so there's a little bit of skipping here and there which is also reducing the rate at which I can wear in the new chain by reducing the amount of force able to be applied when setting off.

Whilst there is probably a lot to be said for frequent regular small drivetrain-cleansings the advantage of leaving a largish gap between proper cleans is that most of the gak on the jockey wheels can be scraped off as a single large oily plaque.