This is likely a "dumb" question. We had a spot color file in PS that totally effed us, all channels where thresholded but it left certain areas at 99% K. Couldn't see it on monitor in PS, couldn't catch it in pre-flight where we use a a temperature map to catch this stuff so it made it all the way to imaging before someone catches it. It's a little maddening since our pre-flight is thorough and threshold = 0% or 100% K, not 99.....
I can adjust our temp map to light up 99% or less but I'd rather this get taken care of in PS v. making it to AI for final output.
It's really the same fight I'm having these days with really poor preview ability in channels.