OK st.cichlid, what of you wanted to tell the sex before the plec was fully grown??!! I don;t think commons don't get to 18" as a rule incidently. I think 12"-15" is more the norm.
What was said about the big head thing is true. The head of a male will be wider when viewed from above than it's abdomen, unlike the female who's abdomen will be wider than the head. I also know that some plecs at least (not sure if it's all) get small odontodes on the tail and sometimes cheeks. Look like little spikes.