Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Call emergency services if you have severe difficulty breathing, cannot speak in full sentences, become confused or unusually drowsy, or notice blue or grey lips or skin.
Emergency help is also needed if your reliever inhaler like albuterol or ipratropium is not helping or breathing continues to worsen despite following your asthma action plan.
And for anyone, someone who:
Seek a prompt review if symptoms are becoming more frequent, waking you at night, limiting activity or not responding as they normally do.
New symptoms after a chemical spill, fire or heavy fume exposure also need prompt assessment, even if you initially feel better.
Arrange a routine medical appointment if you develop repeated wheezing, cough, chest tightness or shortness of breath, especially if symptoms are worse during or after work.
Mention any improvement on weekends or holidays and any exposure to dusts, fumes, sprays, animals or chemicals.
The article also says
Move away from an active spill, smoke cloud or chemical release if you can do so safely.
Do not re-enter a hazardous area to help others without appropriate training and protective equipment.
Every sign above is quoted from the sofidex article on Occupational asthma. If you are not sure, err upward — it is always reasonable to be checked.