1968 S Proof LINCOLN CENT

It's a beauty! Your question was whether the toning is authentic. This is very tricky. You can get this effect by holding the coin over a stove and applying heat. The most reliable way unfortunately it's to send it off and have it graded, which can cost upwards of $40. And then you're at the mercy of the grading companies because sometimes they get it right and sometimes they don't. You're now in the realm of subjectivity, where it gets kind of philosophical and a matter of opinion as to where black becomes gray becomes white. I'm afraid my answer is not gonna be very satisfying to you. I have gone to ANA summer seminar and you can actually show a coin like this to the worlds most foremost experts and they will argue over whether the toning is authentic or not. There's a good book on the topic, a controversial one called coin chemistry" by Wermer White. The moral of the story here is the best reason to buy a coin is it because you like it and not because you care what anybody else thinks of it.