Balance is tough when we don't have a ton of time to play test. I don't think my game is too hard since I've seen people beat it, but there are definitely some things i'm going to change once the rating period is over.
It depends on the game of course, but my strategy was just to play each level over and over again. Even that is not ideal though, since as the creator of a level in a game you know the level very well while new players are just seeing it for the first time. Things that seem easy for you end up being difficult for other's because of this.
At the end of the day I think you gotta play test as much as you can and get it as good as possible then once it's released you'll figure out the things you missed. Unless of course you have people to test it out for you, then get as much of that as possible beforehand.