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If you already have the Lewis Dot Drawing of the molecules correctly completed, you can just count the number of electrons around the atom by counting up each electron of the lone pair as one electron and each bonded pair as one electron as well ie 1 for a single bond, 2 for a double bond, etc. and compare that sum to the number of valence electrons you know the atom should have according to the periodic table to get the formal charge.
Either way is fine if you get to the correct answer, but on an exam I think it's better to write out using the formula because in doing so, I believe it'll get you some partial credit if you were to make a mistake when drawing a Lewis structure or a resonance hybrid.
First you should look at the periodic table and determine how many valence electrons that the element should have. Next, count the total electrons present in the form of both lone pairs and bonds. Then compare this to the ideal number of valence electrons that the element should have. If there are more electrons, the formal charge will be negative. If there are fewer electrons present, then the formal charge will be positive.
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