How would we know if time travelers are visiting the present? You could look for clues — but the best indication might be found on the internet. Unfortunately, two researchers tried to look for clues to time travel online... and came up with nothing.
It's an unpublished study that appears at the Cornell University Library's arXiv pre-print archive, but it's interesting nonetheless. Similar to looking at old photos or newsreels for chronological inconsistencies, or the case where a man named John Titor told the Internet he came from the future, researchers Robert Nemiroff and Teresa Wilson searched the web for prescient information not previously available. Technically speaking, then, this wasn't a study delving into the physical plausibility of time travel; rather, the authors tried to prove time travel by uncovering anachronistic (i.e. chronologically impossible) data on the Internet.