My granddaughter has had asthma since she was a baby. An attack starts with a faint breathy, rattly sound coming from her throat and then advances to an obvious tight sound that indicates she is having difficulty getting air in and out. If left untreated her breathing becomes increasingly more rapid with a loud rattle. Her face becomes drawn and whitish in appearance. If she doesn't get treatment you can see the spaces between her ribs going in and out. If we've waited this long she is in immediate need of emergency room attention.
Unless you've already had a diagnosis and a rescue inhaler the child should go to the ER once you're aware she's having trouble breathing and you know it's not only the result of a plugged up nose.
Unfortunately, one time her mother waited to the point that her breath smelled of acetone which meant that her body's cells wear breaking down. This is life threatening.
My granddaughter's asthma, as a baby, was always caused by a cold. Thus she had a definite rattle sound to her breathing. We learned to always give her her rescue inhaler at this first sign when she had a cold. Babies may rattle with a cold and not have asthma but not my granddaughter.
A clue that the rattle is asthma is that the babys breathing difficulty increases as does her coughing. At some point the coughing stops because she doesn't have enough air.
You can place your ear against their back and hear a rattle in the lungs if the asthma attack has continued for awhile. (perhaps 10 minutes) The length of time it takes for sounds to be heard in the lung by your ear depends on the seriousness of the attack. As we became more experienced I realized that I didn't need to wait for sounds in the lung to know it was asthma.
My granddaughter is now 9 and recognizes the onset of an asthma attack before anyone else can hear her wheeze. I recognize it by the drawn, tired expression on her face. Her complexion also looks whiter and the circles under her eyes darken. If she tries to walk she feels and looks short of breath. She also coughs.
I think that with a baby or toddler the difficulty in breathing always increases within a short period of time to the point that treatment is necessary. I did spend time with a teen who refused to take her medication and she wheezed with any exertion which meant she was audibly wheezing and frequently coughing most of the time.
My granddaughter had several trips to the ER before we were able to give her the right treatment quickly enough. She was unable to use an inhaler at 8-10 mos and needed a nebulizer. I had had no experience with asthma when this first started and perhaps, like you, didn't recognize the symptoms. We should have purchased a nebulizer after her first attack but no one suggested that would help keep her out of the ER. Each time she had difficulty breathing we went to the ER where they used a nebulizer.
If you see a baby or toddler having difficulty breathing even tho their nasal passages are clear, take her immediately to the ER. An asthma attack is different than difficulty breathing because of just a cold. You can see the difference once it's happened.
Addition: I agree with Megan except you need to go to the doctor before the distress is strong. The symptoms of stomach sucking in, blue tinge, and nostrils flaring is the extreme. At this point the child or toddler is not getting enough oxygen to maintain a healthy brain and body. A short period of this apparently does not result in permanent damage but why risk it? And.....the baby/ child is feeling the stress and is most likely scared.