How can I find out if a particular patent or patent application is still valid?
Legal status information (i.e. where and whether a patent is valid) for European and Euro-PCT applications is available via the EPO's free online European Patent Register, where you can also access the publicly available parts of the application file, including the grant stage (and including any opposition/appeal procedures).
Once they have been granted, European patents revert to the national patent offices of the designated states. You should therefore contact the relevant office for any post-grant legal status information you require.
Legal status information can also be accessed via the "Legal events" tab in Espacenet. This information is based on data the EPO receives from the national patent offices.
For more information you can also contact the EPO direct at support@epo.org.