If you are celiac or something you just have to learn to be extremely vigilant. If you are not, then a mistake like that isn't the end of the world. I'm pretty sure some of the dishes I tried at a potluck last night had some flour in them. It's not going to kill me, but I did notice that my gluten-free friend did not eat any of the potluck last night, so that's probably how she ensures she doesn't make a mistake.
Female, 5'3", 48, Starting weight: 163lbs. Current weight: 135.
Starting squat: 45lbs. Current squat: 145 x 4.