Ynot asked: What determines in which class or what level of sotapanna one is at?
Like Lal mentioned it has to be determined by the level of understanding of aniccha, dukka, anatta etc. It is simply the level of understanding of them, that one is categorised as a sotapanna or sakadagami etc.
In the Puggalapannatti, these 9 types of people are explained. The explanation is based on the time frame that each person attains nibbana, rather than the reason a person is a specific type.
Only a Buddha, will know and be able to explain the differences in these types and how/why they fall into each category. So even if this was explained (in the tripitaka), we would not be able comprehend and decide which category oneself or others will fall into.