Service historys are practicaly useless without the invoices to prove it, a friend of mine who owned a frachised dealership caught one of his staff bringing service books in & stamping them, evidently it were a nice little earner!!!
You cant, unless the previous owner gave you receipts showing what had been done.
If you cant find out if the cambelt has been changed, have it changed anyway as an insurance measure, as the last thing you want is it breaking. Dont touch the head gasket unless you have a problem.