It's completely invisible, but the distinctive smell of electrical discharge after photocopying is a tell-tale sign of ozone in the air.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/new-research-finds-ozone-smog-may-cause-asthma
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/medical-express
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/air-pollution
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/health-effects
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/ozone
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/respiratory-diseases