Yeah I can feel it pulsate when I put my hand on it to check it - eewww - glad the hair covers it!! Tanya the MCH didn't say what symptoms I should be looking for but I looked up dehyrdration in Kid Wrangling and Baby Love and they mentioned runny poos, dry skin or cold clammy, dry mouth lips and tongue, face and eyes sunken, less wee and few nappies than usual, darker coloured wee, foul breath AND sunken fontanelle, and from Baby Love
a sunken fontanelle is not a sign of impending illness in a healthy baby unless there are other signs and symptoms that there is something wrong.
Doesn't mention what those are either though, so there it is! Mmm this mothering gig is tricky!