i'm seeing that it comes down to one's motivation for doing good works; the state of one's heart. If you are volunteering at the soup kitchen because that is expected of you by your peers, or even if you just feel that that is what is expected of you, then that is salvation by works (SBW), and this is not some hidden, subtle thing, even if it is hard to describe; anyone who volunteers can attest to how easy it is to recognize the people who are there because they have to be, people "doing hours." You don't even have to ask.
What is in others' hearts becomes revealed pretty quickly; it is what is in our own that is hidden.
Faith without works is dead; so what is faith? Verbally agreeing with the pastor? But obviously some are truly converted at the altar. But for many others, the majority i think, finding faith is a fragile process. Faith--like salvation, i think--is not an on/off thing, 'either you have faith or you don't,' even though it is often portrayed that way. Faith must be built, for most people. Faith can be lost. It can be misunderstood, and it can be misapplied.
Either you have faith or you don't (

), and your works are a reflection of your faith.