Learn how to differentiate metadata associated to a picture and metadata from the web and a picture that comes directly from a smartphone.
(Metadata is information associated to photos and videos: time, format, date when the picture was taken.)
Use specific software. Tools like http://tineye.com are useful to check if a photo has been modified by browsing the web for similar copies.
New tools are coming up that can add watermarks or signatures to photos and other documents. One example is https://guardianproject.info/apps.