How do I test my wallet backup?
You can configure Bitcoin QT to use a specific data directory, but it only looks in that place for wallet.dat.  

I would (1) close Bitcoin QT, (2) move the existing [data directory]\wallet.dat to another folder (3) copy your backup into the [data directory], and (4) reopen Bitcoin QT. 

Don't overwrite your existing wallet.dat with the backup!  Step (2) is pretty important in case there's a problem with the backup file.