The truth is, we can't know. We've tried to make it the best paper it can be: an introduction which describes previous research, how this study contributes to the literature, and what the aims of work are; a clear description of the the methods used; findings with a logical structure, based on and drawing on the data and our analysis; and a discussion which highlights how our findings relate to, and add to what's already known in the field.
But still, I know that despite all this we cannot forsee everything that reviewers will identify as the paper's weaknesses, nor guess ahead of time what might make the difference between a published or rejected paper. Somehow though, there comes a point when you 'know' that a paper is ready to face its critics - that you've reached the end of what you can do to improve and refine you work. It's the best you can do now, and to improve it you need some critical distance, and the impartial, honest opinion of a cold reader who's not spent the last few months working away at it. Critical friends are of great value, but at some point it has to be submitted.
Nervous anticipation is just part of the process I guess, and I'd rather have that than a paper that's been 'almost ready' to submit for as long as I can remember, but which is never quite good enough.