"Смотреть на человека, смеясь (над ним)." Look at the person, while laughing (at him.)
"Смотреть на человека, смеющегося (над собой)." Look at the person (who is) laughing (at you, etc.)

I think that's right, anyway

I used to think of the present tense adverbial participle as meaning 'while (verb)ing', and the past tense one as 'having (verb)ed'; if I went back to thinking in these terms, I think I could avoid confusing the adverbial and adjectival forms. Thanks for the nudges in the right direction.