A nucleophile is electron rich, and an electrophile is electron poor. If you're looking at a molecule and you want to know which part is nucleophilic, look at the dipole moment. The part that is most negative will be the nucleophile. For instance, something like ethanol will be nucleophilic at the oxygen atom, or an alkene would be most nucleophilic at the double bond.
On the other hand, if you want to find the electrophilic part of an atom just look for the positive part of it. For example, ethanol is electrophic at the C attached to the oxygen atom since the oxygen atom is electron withdrawing(at least in this case).