This is probably because we all have a certain preconception of how bad our breath smells, and when we come to score what we smell, we are heavily influenced by our mindset, regardless of how bad the smell actually is. Several years ago, a Japanese company came out with a little bad breath detector that looks like a small powder box, mirror and all. But when you open it, there are little buttons and lights that go on and off. You blow over a small grid, and within several seconds it gives you a score. However, there is a serious question as to whether these small instruments are at all accurate.
Most people have bad breath at one time or another.
The best way to find out if your have it on a regular basis, is to ask someone close to you. Provided that they love you, and that they have a sense of smell, family members will find a way of telling you the truth . You can also ask a very close friend.