Four things people often call an “AI signature”
Portable metadata is information attached to a file. It may include a generator name, XMP field or C2PA Content Credential. A C2PA credential is signed and can be validated by a compatible tool; a plain text generator field is not the same thing. See Content Credentials explained.
An embedded watermark is part of the image, video, audio or text signal. A vendor-specific detector may recognize it even when ordinary file metadata is gone. SynthID is an example. See invisible AI watermarks.
A platform label is a decision and display made by a website or app. It may be informed by a file signal, a user disclosure or records held by that service. You cannot infer which source it used just by looking at the label. See why an AI label may remain.
A classifier score is an estimate based on patterns in content. It is not a signed record of origin. A classifier can be wrong, and its coverage depends on how it was trained and tested. NIST's overview of synthetic-content transparency discusses these approaches and their limits.
Which question are you trying to answer?
If you want to know what travels with a particular file, inspect its metadata. If you want to validate a signed history, use a C2PA validator. If you want to check a specific vendor watermark, use that vendor's detector where one is available. If you need to make a high-stakes judgment about origin, use multiple independent sources and context; no single negative result establishes that a file was made by a human.
Free AI Wiper serves the first question for supported files and text. Its test method states what it checks and what it leaves outside scope.
Check a social media AI label
A platform badge needs its own explanation. Start with AI labels on Instagram, Facebook and LinkedIn, then follow the guide for your post type. For video exports and cover images, use the Instagram Reels checklist.