Hi,
you can determine the effects by looking at the structural formula. Determine which resonance structures are possible. The possible structures show you, if the group is able to function as an electron pair acceptor (-m) or an donor (+m). I-effects could be determined by electronegativity. The electronegativity of one element compared to another can also help you to determine, which i-effect is dominant. Normally, m-effects dominate compared to i-effects of the same group (and also by comparing different groups). F, Cl, Br and I are exceptions.
