As it applies to who exactly is posting here, I think it's a valid question. It's an online forum- I could post on here saying I'm a former playboy bunny who discovered she was a talented mason, and how would anyone verify that?
As far as the guys you see driving around, that's hard to quantify because there really are no universally recognized standards for what to call yourself. Most states have laws governing whether or not you can use the term "landscape architect," but I don't know of any that have practice acts regarding landscape designers. I would think, though, that the unqualified ones would be easy pickings, as long as *you* know what you're doing.
Anyhow, to answer your question- I do this all day, every day, and have been in the industry for fifteen years.