Many elements specifically trasition elements have exception in thier electron arrangement in thier shells. therefore it is not unusual for Zn to behave in that way. Now for metals with 1 charge type such as your example Zn, you can use the electronic configuration, if you know it...
Zn electonic configuration: 1s2 2s2 2p6 3s2 3p6 4s2 3d10
However this way has some rules as to where electrons go first to which orbital, As you can see the 4th s-orbitals fills before the 3 d-orbital, but Zn will lose the 2 electrons in the s-orbital first, therefore will form a 2+ ion.
I suggest not using such a way with transition elements because it can get confusing, and when reaching the f-orbitals it can be difficult.
BUT what you can use alternatively, is that you can recall some of the compound Zn forms such as ZnO, therefore, since Oxygen needs 2 electrons, then Zn must provide 2 electrons, hence Zn forms the ion Zn2+.
Elements with several charges such as vanadium, you just have to memorise them, or remember the compounds it forms and find out the ion it forms with such a compund.
This is what i know, i hope it helped...