You can check/validate any stock identifier. But this does not indicate whether the stock actually exists.
An ISIN is a 12-character alphanumeric code. It consists of three parts: A two letter country code, a nine character alpha-numeric national security identifier, and a single check digit.
SEDOL is a 7-character alphanumeric code and stands for Stock Exchange Daily Official List, which identifies securities in United Kingdom and Ireland.
The Wertpapierkennnummer (WKN, WPKN, WPK) is a 6-character alphanumeric code which identifies German securities.
CUSIP is a 9-character alphanumeric code which identifies North American financial securities.