Before every departure one of the pilots or trained ground staff perform a contamination check from an elevated position to check that the aircraft is free from contamination (as mentioned earlier we may have some frost - up to 3 mm - on the lower wing surface within the wing tank areas. Also a thin layer of frost on the upper part of the fuselage is permitted).
The SAS MD-80's do have the inboard wing heater system in operational use. The system is installed to remove/prevent clear ice formation on the inboard end of the main wing tank upper surface ("cold corner").
Under certain conditions (i.e. the wing heaters are u/s and the outside temperature are below a certain value etc.) a clear ice check may be required. This check is a hands-on check.