Delurking for a quick code snippet. I needed to have AppleScript check to see if a file exists before doing something. Initially I was trying to do this with a bare if *filepath* exists, until ninety seconds of searching revealed something obvious. You need to tell the Finder to do it.
if exists file "Macintosh HD:Users:chrisb:Documents:test file.txt" then
display dialog "it exists"
display dialog "it does not exist"