dd1 was never really a picky eater a ssuch - has preferences, yes, and sometimes they have become entrenched. but she eats a wide range of food within her limited diet, iyswim.
so, checklist. these are signs that somethign might be up. obviously, they could all be down to something else, too.
hyperactiity
sleep issues (late to settle, seeming quite restless even though tired)
giggling/screamin gfor no apparent reason (giggling in particular was a big factor with dd1. she would jst burst into uncontrollable giggles, eg when alone at night before going ot sleep)
excessive thirst
craving/dislike for certain foods (theory is they crave the foods which give them a high, avoid the ones not worth it)
hot and sweaty, especially at night (this can sometimes lead ot thougts of CF, as if digestive issues are present, and very sweaty, these can be factors)
history of glue ear/ear infections
pica
diarrhoea/constipation (either/or, or both)
swollen tummy - lots of bloating and wind
catarrh/runny nose
poor temperature regulation - stays hot/cold longer than usual (also poor temperature awareness, so will go out in the snow in a t shirt and not feel cold)
red ears/face (typical "english rose" complexion - pale with flushed cheeks, very rosy)
dark shadows under the eyes - really dark smudges, looks like hasn't slept in a week, even though you know they have
pale, pasty comlexion (looks ill, even though you knw they aren't)
aches/cramps/tiredness (lethargy - seems to tire more easily than peers)
history of allergy in the family (inc asthma, eczema hay fever etc)
gut issues in the family (crohns, coeliac)
now, obviously this list is not exhaustive, nor diagnostic.
but if you get enough ticks, it might well be worth doing a sunderland test - as spinkle says, another thing to rule out, and no harm doen (other than the stress of collcting a urine sample! but they send a kit to help with that)