I guess you mean acid group ions...?
the way I learned them was to remember all the acid formulae (wasn't my choice really, I just HAD to know them) like HNO3, H2SO4,H2CO3 and so on... and then check the number of ionising hydrogen atoms attached, for H2SO4 its 2, so the valency of SO4 ion is 2, for HNO3 there's only one H atom, so valency is 1 and so on.
If you don't want to remember the acid formulae, I'm afraid you'll just have to remember the valencies or check for them in a data booklet of some sort.
You can also count the charges, eg CO32-, therefore valency of 2.
Or you can calculate it from the salt formed (this is imho a bit more complicated than the H or charge-counting way) because you need to consider the valency of the [metal] in the salt. it's easy for group 1 metals, as they have valency of 1, so you count them just like hydrogen atoms.
But see other valency metals, like calcium. example: Ca(NO3)2 Ca has the valency of 2 and it needs 2 NO3- ions (you can say thet from the formula), so you divide 2 by 2 and you get the valency of NO3 which is 1.
another example: aluminium sulphate Al2(SO4)3, you have Al - valency of 3, so there are 3 SO4 ions. You know that this relates directly, so you can say if there are 2 Al atoms, the valency of SO must be 2.