I downloaded Scid vs PC to analyse my games and I want to check what version of Stockfish is included - how can I check if it is Stockfish 6?
Double-click on the Stockfish executable, type "uci" (without the quotes) and hit Enter. You should get some output, near the top of which should be a line beginning with "id name", and this line gives the name of the engine, including the version number. (You may need to scroll up to see the "id name" line, if there's a lot of output.)
The same method works with any UCI engine.
Note that the Stockfish executable supplied with Scid vs PC is in the bin\engines\stockfish subdirectory of the Scid vs PC installation directory. The version of Stockfish included in my (recent) copy of Scid vs PC identifies itself as Stockfish 240813 64, which is a bit odd (perhaps it's a version compiled from source by the developers of Scid vs PC), but the file date suggests that it's roughly the same as Stockfish 4.