My feeling about providing "free" design advice is that I am sharing what I have learned over the years. I know what works and what doesn't, what makes good design and what shows when someone has too many choices and wants to try them all out.
Yes, it is possible that someone will use my ideas and profit off of them. It has happened in the past and may in the future. However, I am invested in good design and I would rather provide some ideas and suggestions to someone "off the clock" as it were, rather than being miserly with my knowledge.
You will get a sense of whether or not someone is trolling professionally for design advice. You will also be able to discern when they just need a little nudge here and there to help them out. It is always up to you, however. Don't feel pressured to part with your knowledge and experience. After all, you earned both the hard way.
Some of the advice I provide may not make me any money right now, but down the road that same person may be in a position to pay for my design expertise. Even if that doesn't happen, good business relationships aren't forged by holding all your knowledge to yourself.
Sharing is good, and good business.