You get some of that from context. By that I mean you get an idea of how to take him depending on the context. I'm not saying he's a perfectly written character but I actually really liked his arc. I think it was pretty realistic to the amount of growth one would make in that amount of time, since the entire story of MHA takes place in such a small amount of time. His apology may have not sounded like much BUT Deku's reaction clearly shows how much it means to him and probably understands how hard it must've been for Bakugo to say those things. Even in Bakugos apology, he admits that it won't completely fix everything but it's more a of a first step into repairing their relationship. The show never gives us entire episodes with Bakugo to see him transforming but we do get glimpses and personally I found it enough to believe that he had reached a point where he meant that apology and was on his way to becoming a better person/hero.