Depends how much you out in, and how much diesel there was to mix with it.

If the tank was substantially empty, and you filled it with petrol, I think you would know - in all probablity it would stop a mile or so down the road. If you put a gallon in, realised what you had done, and filled up with diesel then there may be no effect at all.

I once filled up a Scorpio 2.5 diesel with petrol - that had an old-tech VM boat engine, also fitted to Range Rovers once upon a time. It ran, sounding like a bag of spanners. I topped up with diesel as space allowed and it seemed not to have any lasting effect. Modern CR diesels with high pressure fuel pumps etc would likely be a different (and possibly very expensive) case.

One concern that people legitimately have is whether a second hand diesel car may have been misfuelled without them knowing. Quite possibly there may have been damage.