Something like checking the directory listing times as well as opening random Lightroom projects just to “feel the difference” is likely to be your best barometer here - although the speed of loading files onto and off of TrueNAS may be a good check to ensure that things are being “split up” between SSD and HDD. If the files are properly sized and separate then you shouldn’t see nearly as much of a dip in speeds between the main file copy and the small sidecars.