I favor the question of whether you believe in "sin" over an external test of whether you do X or Y, because it reflects more clearly what a person actually believes. I think that many "non religious" are really religious people who for various reasons live in sin.(on this matter read my post "searching for the religious true self"). THis is quite evident by the fact that so many "religious" Jews commit some quite horrendous sins without seeing a conflict to being labeled religious.
As long as one has that nagging feeling of guilt stemming from an unshakable belief that God isn't happy (as opposed to guilt stemming from their parents not being happy) he is a religious, if sinful, person.