In SharePoint, you can check the effective access of a user or group against a site, document library or list.
To do this, for example for a Document Library:
- Browse to the Document Library you want to check if a user has access.
- Click Gear, then Library Settings.
- Click More library settings.
- Click Permission for this document library.
- Click Check Permissions.
- Enter the name of the person or group that you want to check permission against, then click Check Now.
- The results of the permission check will reveal what rights the account has against the site, document library or list, and the principal that grants the access through.