Actually its possible to differentiate a spell a bard shares with a cleric because almost always its not truly the same spell its just a near impossible to discern analogue of it.I already have. The bard rules say that bardic spellcasting involves music. Clerical spellcasting doesn't involve music, though I suppose a cleric could play something while he prays and use a sung prayer. Also, with the exception of those few spells that bards can take from the cleric list through their class ability, their spell results will be very different from clerical spells. For someone who knows what to look for, it will be very hard for a bard to mimic a cleric effectively, and even harder for a cleric to mimic a bard.