I used test strips on a semi I owned that would tell me the ph balance of the coolant. Then if it tested corrosive you would add pencool to even it out. The previous owner learned the hard way by not checking it and he had to put in new sleeves. I beleive not sure though that you should use the test strips even if you flushed and changed just to make sure its ok. Somewhere I read the subaru engines had massive trouble with this because the battery was too close to the radiator and would somewhat charge the coolant, make it highly corrosive and hence all the head gasket replacements at 80k.