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 HNO_{3}, H_{2}SO_{4},H_{2}CO_{3} and so on... and then check the number of ionising hydrogen atoms attached, for H_{2}SO_{4} its 2, so the valency of SO_{4} ion is 2, for HNO_{3} 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 CO_{3}^{2-}, 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(NO_{3})_{2} Ca has the valency of 2 and it needs 2 NO_{3}^{-} ions (you can say thet from the formula), so you divide 2 by 2 and you get the valency of NO_{3} which is 1.

another example: aluminium sulphate Al_{2}(SO_{4})_{3}, you have Al - valency of 3, so there are 3 SO_{4} ions. You know that this relates directly, so you can say if there are 2 Al atoms, the valency of SO_{} must be 2.