Everytime I've scouted the metagame, I've basically ended up with "shit, it's a little bit of everything, with only slightly more X or Y". When the number of opponents you play is a large proportion of the size of the turnout (most tournaments smaller than a PTQ), it's hard to really get a handle on a metagame.
For large tournaments, if you test well enough with the proper people, you'll be able to figure out what you need to play.
For small local tournaments, you should know what everyone will be likely to play, and thus, plan accordingly.
For small non-local tournaments, you just need to play the best deck and hope.