In Serbia, it is likely you are dealing with either hollow (multi-cell) clay block or just solid walls depending on the age of the structure). If is is very old, you could find a solid wall. It is difficult to tell since both walls are are very solid and hard because of the plaster covering on the walls.
Drilling a small hole will give you a clue of the material in the wall (clay block will be reddish and in some areas yellowish and concrete will be gray). The drilling rate and ease should tell you something about whether is is hollow or solid. Many walls are either solid concrete or solid masonry depending on the age.
Since you are dealing with permanent structures, it is not as easy as people in the U.S. that are more acustomed to dealing with more temporary wood framing.
How old is the building and what do you know about the other buildings in the area? If it is an apartment or flat that is a building over 3 stories, the wall may be concrete. Tito built a lot of Russian-style precast plants after 1945. If it was built between 1960 and 1985, I could be gas concrete (Ytong type) that is solid, but easy to drill into.
Around stairways, you could find a variety of different materials or construction methods.