Google's SynthID Detector is now available worldwide in English: upload a photo, video, or audio file to check for an invisible AI watermark. It covers Google, OpenAI, Nvidia, and Kakao content; Apple Intelligence support is coming "soon."

Limits: it only detects watermarks embedded at creation — a clean result doesn't prove a file is human-made — and it can't tell AI-created from AI-edited. Sign-in required. Handy for checking dubious forwards in family groups.