Take your finger (any one, whichever is most comfortable) and gently rub the far back of your tongue, the further back the better, you should try to rub as far back as the end of your tongue. Smells bad? That's what your breath smells like.
Take a plastic cup and breathe into it as if it was all crinkled and you were trying to blow air into it to puff it up again
Take the cup and hold it up to your nose.
If your breath smells bad, brush your teeth again.
Lick the back of your wrist and wait 5 seconds.
Smell the back of your wrist. If you smell something bad then your breath smells bad, if you smell nothing then your breath smells fine.