(1) Time Travel Exists
(2) People are capable of traveling back in time
(3) People are traveling back in time; then
(4) No evidence would exist of said time travel; because
Any responsible time travel program of the future would have policies to mitigate world ending things like paradoxes through commonsense rules such as not leaving smoking-gun evidence in the past informing those past generations that people from the future are visiting them. If there was no such oversight, and the person from the future was just "Doc Browning" it then there would already have been documented evidence since any oversight by any authority form any time in the future would most likely intervene in this scenario as well.
Thus, time traveler who traveled back in time, upon their return, would be able to verify that they did not leave any such evidence behind by simply checking their historical archives. If they did leave behind evidence that was noticed and documented, then using said records, he would know what said evidence was that he needs to retrieve, as well as when and where in time to he needs to go to retrieve it. Upon the time traveler's return, the disappearance of the record from the archives would verify that they were successful. This of course makes the assumption (that is quite common in science fiction) that the time traveler's memory is somehow protected during time travel from the effects and changes caused by his time travel allowing him to remember how things were before he changed them.
Lastly, given human nature, if said time traveler was researching the historical archives, and happened to come across an article such as this one, would he not be tempted to go back in time and mess with researchers by leaving some form of "obvious evidence" (obvious from a future perspective / but not out present) making them look foolish for the amusement of the time traveler and his friends.