Some planes do not have class differentiation(eg most of the British Airways domestic arilines and even Indigo/jet airlines that operate in India. (in UK certain trains do not have 1st class so you need to check before bording as tickets are open for any train if you buy return. If you buy busness class tickets that include domestic airline most probably you may not get business class for that journey. So ask from travel agent or purchasing body.
Sri Lankan business class is very unprofessional . My luggage was lost in the transit by the Airline but only paid Rs 5,000 , which is the same as economy class. Their Customer Service for business class is very poor . Serendib lounge is good but even the seats are not able to recline and televisions are not working and even if they are working sound quality is poor. So Sri Lankan airline business class is not good compared with other airlines.