I'd say it's pretty likely. I don't know diesels that well.
There are a few ways to test without using any type of gauge or meter. Downshift and take your foot off the accelerator. Let engine braking continue down to idle. Pay attention to how the engine braking feels. If shortly before idle it lets off, as if you had suddenly put it in neutral or stepped on the accelerator, then you had DFCO.
Another test, which may be more risky, is to turn the engine off while engine braking. If you feel the engine braking increase, you did not have DFCO. If it feels the same, you had DFCO.
With no throttle, a diesel might feel different for all this stuff, so I'm not sure that my advice is correct for you.
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