--Bore scope is the only way to see it.
--Increased pressure/velocity with a given standard load (when nothing else has changed, shows it
--feel it, like BH posted above.
My first indications are increased velocities which in turn give me vertical where there was none before.
As of now, I go after carbon proactively.
GM Top Engine Cleaner (Shooters choice or Butch's should work as well) with brushing to get most of the carbon, followed by a wipe-out soak to get the rest and the copper work for me.
Along with that, I scrub a patch of JB paste thru every ~100rds or so to keep ahead of the carbon formation.
If you think you have carbon issues(veloities increasing and groups suffering) check the velocity...then scrub with JB and check again...if the velocity dropped back down where it should be, you had carbon.
These links may help: