The Google Image reverse search function
Google allows you to perform text searches, but not only text searches. Using the Google Image tool, you can also find out whether an image you have has already appeared on the internet. This can be useful in a situation where a source states, for example, that a photo is very recent.
To perform this verification, go to the Google Image site. Take the photo from your computer and insert it into the search bar. Google will then show you all the web pages containing that photo. You will then be able to check the publication dates of the articles that contain the image.
If Google is unable to find a precise answer, this tends to indicate that the photo is indeed recent. It is important to note, however, that this tool only verifies the fact that the image has never been published online. If you are still in doubt, perhaps you should continue your investigation using more traditional methods.
The EXIF and IPTC data of the photos
Image files, like most other file types, sometimes include hidden data: those are called metadata. These data are usually technical: model of camera used, shutter speed of the device, etc. Having said that, you can find very useful data for determining the origin of the picture.
The EXIF or IPTC data sometimes contain information such as the name of the person who took the photo and the time and date of when the photo was taken. With a little luck, you may even find the geographic coordinates of the shot or the type of modifications applied to the original photo. These are all additional elements that will allow you to verify the veracity of what you see.
Each piece of image processing software has a different way of accessing the metadata of an image. An internet search will likely allow you to find the solution you need. When using Photoshop, press Ctrl+Alt+Shift+J (or Cmd+Alt+Shift+J on Mac) and the metadata will appear.
Spotting a video that has been published previously
Investigating a video is a little more complicated. However, there are some tools that can help you. Amnesty International has developed a tool to help you determine whether or not a YouTube video has been posted on the network previously.
InVid is more comprehensive and more technical, intended primarily for journalists who want to verify images circulating on social media. In particular, it allows reverse searches from videos published on Twitter, Facebook, YouTube, Instagram, Vimeo, Dailymotion, LiveLeak and Dropbox.