I know when William has a tummy ache as he is restless, drawing his legs up towards his tummy a lot or bashing his legs in his cot and he passes wind a lot. Without being too graphic the contents of his nappy also alters from the normal - maybe v soft, maybe too dry..
I have been lucky so far that only cheese appears to give him a tummy ache - which is interesting as I am totally intolerant to cheese..
HTH
Helen