You can check for a file with something like:
NSFileManager *fileManager = [NSFileManager defaultManager];
if ([fileManager fileExistsAtPath:aPath]) { ... }
Depending on what you are looking for, "aPath" might be something like:
NSString *aPath = [[self applicationDocumentsDirectory] stringByAppendingPathComponent:@"Sample.txt"];
or
NSString *aPath = [[NSBundle mainBundle] pathForResource:@"sample" ofType:@"txt"];