the way we knew DS had mild asthma was a wet cough - particularly at night or after exercise. They gave us a low dose preventative (also, use an spacer, as it increases the intake by over 50%) to give at night (and maybe at morning), and after 6-12 months I got a bit slack and missed it a bit and noticed he didn't seem to need it, and hasn't had it in over a year now.

It helped his sleep also, infact the week we started we also had him at a paediatric sleep study to check for sleep apnoea (which he didn't have - and they put it down to asthma)