How do u know if a 3 month old baby has asthma?

Not always possible. At only 3 months old, if your baby is wheezing, it could be viral bronchiolitis or asthma or both. Either way, he/she needs to see the doctor, as both need evaluation and treatment. To make a diagnosis of asthma, it usually requires having recurring wheezing over a long period of time.
Too soon to tell. Coughing, wheezing & labored breathing are the symptoms we think of defining asthma. In a 3-month old more likely considerations are gastroesophageal reflux, a congenital anomaly of the breathing tubes & major blood vessels, laryngotracheomalacia, bronchiolitis & even an aspirated foreign body. If you think your 3-month old infant has asthma have your pediatrician see him/her promptly.
Asthma. Very difficlut to diagnose in a 3-month old. And to have a doctor label asthma in someone so young is inappropriate because then insurance can scam you by calling it a "pre-existing" condition. Best thing to do is keep a diary of cough, wheeze and shortness of breath; also of infections. If breathing is bad, oftentimes best option is simply to treat because we can't really test until 4-5 years old.
You don't . Young babies may develop bronchiolotis or infections caused by viruses, RSV amongst them. Sometimes babies that have had RSV tend to become wheezers and some become asthmatic. Watchful waiting and periodic check ups are in order.