How do I know whether my garage did an oil change or not on my Volkswagen Touran?

I had my Volkswagen diesel Touran serviced yesterday. When I got home, I checked the service book and found that they had ticked the 'Oil change: No' box. However, the invoice included charges for oil and filter. When I rung the garage, they said that they had made a mistake in annotating the service book and that both oil and filer had indeed been changed. I have looked at the dipstick and the oil does appear rather dark. I guess that a diesel engine does discolour its oil pretty quickly even after a short run, so is there any way I can confirm whether it really has been changed please?
Burning diesel puts so much carbon inside the engine that it blackens the oil very quickly. But if in doubt you can have a 'bocle' test carried out on the lubricity of the oil by a university petrochemical lab. You could also have a look at the filter. Old engine grime on it, not changed. Wiped over with an oily rag, not changed. New looking, probably changed.
Answered by Honest John on

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