I don't have my philosophy head on tonight, but I operate under the policy that something's real if:
1) I can poke it and it feels solid, or I can identify a pokeable source for the sound
2) It's something scary/dangerous and other people are running away from it/shrieking about it/ringing the police about it
3) It's so inconsequential that I don't even need to waste my time on it. (I saw three coffee mugs on my desk earlier. Only one was real -- it just didn't matter one iota.)