How to take Snapshot of Restoring Database
Its a bit round about. But first you have the option in the configuration to change the resulting database to 'standby mode'.

This results in a destination database which is grey, and is marked as 'Standby / Read-Only'. You can look at the tables and query it etc.

Take the database offline (TestReplication is what mine is called), and copy the mdf and ldf files. Put it back online again.

Create a new database called 'TestReplicationNew'. Take it offline, and copy the mdf and ldf files from the last step over top of the TestReplicationNew mdl and ldf files.

Take it back online and you have a read write database that you can test works.

If I don't do it like this I get an error,

cannot attach a database that was being restored