When my gasket went the only clue was a bit of oily ploop in the header tank and a corresponding drop in the oil level. No misfires, no external leaks or anything.
They pressure tested the blocks (some engines are more well-'ard than others) and couldn't see any problem.
The dodgy gasket was only apparent when they took the motor apart.
A quick skim and a considerable amount of moolah later (my mechanic seems to charge £500 plus parts and labour, for everything) and it's all tickety-boo (though my water level still seems to drop a teeny amount).