When you say legit, do you mean as in the artist has not just directly traced an already existing picture? Anyway, i wouldnt call someone who simply imitates another artists style completely an legitimate artist even if its not traced or colourpicked. Take for example all my little pony vectors that are being posted, i wouldnt call that legit art!
You can't tell. The internet is outta control, man. What you can control is what you want to look at. If you're just looking at digital fanart of cartoons or games, then the chances are high that it's fake. If you take a look at hand drawings and painting and things like that- much less popular and therefore less copied/stolen, and usually very different from artist to artist- then you'll be reasonably certain the picture is genuine.
Even that is not a given. A week or two ago, I came across someone on dA who took another dA user's traditional art and passing as her own. She also had a tutorial on how to paint, back stories on why she drew whatever piece ... basically, she had her t's crossed and her i's dotted. If she had not been stupid enough to steal art from someone on dA (who is very good, but not well-known), I doubt ppl would have found out.
Easiest way is to usually check to see if their gallery has any actual original drawings. The comparison of skill between them is usually enough to give it away lol. I see a LOT of anime tracings (someone took a screen shot and traced over it) and usually the dodgy line work alone is enough to give it away, and my favourite are people who paint over real photos, the re-painted people always look so awkward and unsure of how their features are supposed to be defined. I can link to the original work all I want really, the scam artists will just hide my comments and block me. I think the best way is just to leave them to their own devices, if they're going to claim they can draw and paint so well there will be a time where they'll be called to prove it with requests from their friends and family.
They're only fooling themselves, and at the end of the day I take solace knowing that galleries and other artists of much higher caliber will be able to tell right away too. They won't ever get too far in the professional art world.
If it looks like a photo I check WIPs and Making Ofs... If not, it may be a drawing. If someone copies exactly the work and filters it a bit its no art. I always use 3 or more reference images, when I draw a person or figure from a movie, game etc. You can see that its not exactly the same. I change lightning and sometimes even clothes.
it's very easy. ask to see some of their line art. it's the only reason why i have a few of my old pencil works in my gallery. all of those are non reference use. all from my imagination. i've recently been using tons of ref, which i found was beyond helpful. at my stage, i'm mixing and matching. imagination and reference. i know in about a year or 2 i'll be able to do mostly non ref work. although i will still use ref because i need that realism in my work. i'm huegely inspired by dan luvisi. also, when using ref, i use a grid, so proportions might be close to exact. if you ask any pro in the business, they hate the questioning of grids. the old masters even used them. back to painting.
"if you ask any pro in the business, they hate the questioning of grids."
Who are these "pros" you speak of, and how do they "hate" the questioning of the grids?
Grids fall under the "legit" umbrella, but I'm pretty sure there are pros who would not be proud of being too reliant on them, if it came to that. And the old masters usually used grids merely to enlarge their own compositions onto a larger surface. They still had to be proficient with regular non-grid freehand drawing as well.