This ^Any substantial mould type conditions enough to result in health issues like that you should smell fairly plainly - especially if you are elsewhere for a day or two and come back it should hit you as you walk into the house or room.
Might be worth checking down the back of anything especially furniture wise that is up against a wall though.
In some cases mould can be endemic in the walls i.e. previous occupant just papered over it and can still cause issues even when not visible.
I already have asthma but developed worsening symptoms one year that led to a chest infection / bronchitis and I couldn't shake it off falling evermore unwell. The house wasn't particularly damp, or so I thought, until I moved the bed and noticed it at the bottom of the wall but didn't appear too bad, certainly didn't deem it enough to cause the issues I was having. Until that is I noticed all the green fluffy mould at the top but hidden behind and underneath the cross bar of my headboard, I'd been breathing the spores in for months.
Definitely worth as suggested by Rroff, moving furniture and having a good look in the most unlikely of places.