or if you know how many characters you've received... just run a simple for loop comparing each element in your pre-stored array with the received message in the buffer...
there are lots of ways & you can keep tweaking this.. eg. ignore case etc 
just google & lookup the refs of <string.h> in AVR LibC which is the underlying C library on top of which Arduino libs sit - there are a rich set of string functions... though these come at a slight cost in terms of compiled program size