A lot of times, I've seen ppl try to smell their breath by cupping their hands over their mouth, breathing into it and trying to smell it. If this actually yields a positive result (actually smells), then u have halitosis (bad breath). Most times, u don't smell anything and u are happy, "no mouth odour".

Try this simple method.
Assuming the back of ur hand is averagely clean, using ur tongue, smear some saliva on the back of ur palm and put it to ur nose to smell.
Do this after the previous method says u are good to go, repeat it after brushing and see the difference.
Thank me later.