Typically if the water gets low the tmperatures will go up a few degrees. There is the
outboard reservoirs that sit on top of a case or beside it etc. If you see it get lower then half then its time to fill up.
EDIT: here is a high rez pic of what a typical water cooling system looks like during a leak test..
Danny if this seems like too much of an undertaking you can go with one of
these outbaord setups
~Bob