i'll try. not sure it will be accurate. i'm not that much into the english language too.
1) Definition: corrosion (simplified) - result/process in which "something" takes electrons from metals, thus new compound is added to a system.
In our cases those new compounds will decrease WC performance as they have lower heat-conductivity than metal itself, and they can restrict the flow of coolant. With aluminium it's quite easy to prevent it corrosion in the system. It's corroded delibrately before. Thin layer of Aluminium oxide (Al2O3) is not soluble in the water, so futher corrosion is prevented. (ANODized Aluminum). What can be done to copper to prevent it's corrosion i don't know (yet).
2) Begining: corrosion will start when there is a metal and something active enough to take electrons from him and the conditions are aproriate for the proocess. In the coolant that runs through WC there are materials that may corrode copper (water itself is one of them and weak one), so other compunds are added to coolant to make conditions unsutable for corrosion.
3) Signs: Corroding copper will lose it's brightness, aluminium also. It's impossible to see unless you have water blocks with transperent cap. ... i realy do not have any smart sugestions about discovering corrosion. The only thing i can think of is to check if the performance of your systems drops. If it does corrosion may be one of the factors. Again, it's not smart, because so we wait for the problem and not preventing it.
hope i don't have any mistakes and others will add the info i don't know. Check these links:
http://www.overclockers.com/articles609/
http://www.amdmb.com/article-display.php?ArticleID=196
p.s: i don't know if process in which two metals are involved is called corrosion too, although there is some similarity. halvanic pair, electrolisys - these are the terms. anyway check the links.