I found this question, which has an answer, but facebook changed the token format since then, now it is something like:
AAACEdEose0cBACgUMGMCRi9qVbqO3u7mdATQ
The official graph endpoint for inspecting access tokens is:
GET graph.facebook.com/debug_token?
input_token=[user_access_token]&
access_token=[app_token_or_admin_token]
Example response:
{
"data": {
"app_id": 138483919580948,
"application": "Social Cafe",
"expires_at": 1352419328,
"is_valid": true,
"issued_at": 1347235328,
"metadata": {
"sso": "iphone-safari"
},
"scopes": [
"email",
"publish_actions"
],
"user_id": 1207059
}
}
app_token_or_admin_token can be obtained using the Graph API call:
GET graph.facebook.com/oauth/access_token?
client_id={app-id}
&client_secret={app-secret}
&grant_type=client_credentials
The debug_token endpoint will fail if that user_access_token doesn't belong to the app that generated the app_token_or_admin_token.
Relevant facebook documentation:
Inspecting access tokens: https://developers.facebook.com/docs/facebook-login/login-flow-for-web-no-jssdk/#checktoken
App Tokens: https://developers.facebook.com/docs/facebook-login/access-tokens/#apptokens