A code snippet written in Cocoa (Objective-C) that detects if an application was downloaded from the Mac App Store, that returns as a boolean.

MASDetector.m

- (BOOL)isFromMAS:(NSURL *)appURL {
    bool mas = NO;
    if(appURL != nil) {
        mas = [[NSFileManager defaultManager] fileExistsAtPath:[[appURL URLByAppendingPathComponent:@"/Contents/_MASReceipt/receipt"] relativePath]];
    }
    return mas;
}