The first thing you can do is just ask them. For 20 years, parliamentarians have been asking, “Are you a registered lobbyist, yes or no?” I would guess that most people would know if they should be a registered lobbyist. If their answer is, “No, I'm not being paid to do this”, that's the end of the conversation right now.
The second point that I would make in response to what you just asked is, Parliament did a really good job of articulating some of the things that require registration. The one that could benefit from a little bit more parliamentary decision is what a change in policy is. Depending on where you sit, a change in policy could be very broad. For a piece of legislation, I can figure out what that is. There's a website that lists all the statutes. If it's about advancing a new piece of legislation, I can figure that out very quickly, but a change in policy I think could have a little bit more definition lumped into it.
The other thing that could help Parliament is a preambular statement about what lobbying is or isn't. To specifically say that the work of a member of Parliament working with his constituents does not constitute lobbying or shouldn't constitute lobbying has no effect in law, I think, but does give guidance to the commissioner to say what the intent of Parliament is.