I don't own a challenge cache, but I found one yesterday and noticed something.
In the log section, right beneath the username there is a "Checker result", marked with a Check sign and the timestamp if the user fulfils the challenge. Also see appended picture.
I am a PGC Premium Member and have the Tampermonkey script from PGC for Geocaching website enabled. I don't know if it is automatically checked or not, there also seems to be a slight delay in time (log date and checker date). But there is kind of a tool you're looking for.
For reference: this is the cache: https://coord.info/GC5FCEN