It is not completely certain how long people remain infectious, and there needs to be a careful balance between keeping people away from work when they might infect others (particularly if they are healthcare workers) and with keeping so many people off work that essential public services are compromised. There is more justification for the latter if the virus is more dangerous - but fortunately swine flu appears no worse that ordinary seasonal flu.
Many hospitals are asking staff with flu to stay off for 7 days. If staff stayed off for 14 days there would be much more risk to patients who need essential treatment, from staff shortages.
The easiest way to confirm swine flu is by nose and throat swabs by PCR, but this is only recommended now for patients admitted to hospital. Clinical diagnosis by symptoms is quite accurate with so many cases now.