It's like this
you work out what state they are in, that is if it is insolubal in water (it is solid) and hence all the atoms in it won't be spectator , same for gas or liqueds.
But if it is soluble, that is , if it ionisises, and it's actually in water (that is it's aqueos) then put it in it's seperated form, and if the ion remains the same on both ends of the equation (hasn't been added or substituted) then you cancell out.
it is a "spectator ion"