If Sunnah is independent of hadith, then how does one check what if someone is doing bid'ah (unsubstantiated addition to the corpus of Islam.) or not?
When we try investigating an issue for example the question of celebrating milad (birthday of the Prophet sws) we continue studying the sources in the earlier generations. We go back step by step. We study all the material and works that are used as a source of history. Then after a couple of generations we find that the concept of milad was not there. It shows when it was introduced and by whom. The ahadiths are one of the most important sources of knowledge regarding the views and understanding fo the earliest generations of Islam so are the commentaries on the ahaidth, the Qur'an, the commentaries on the Qur'an, the fiqh works, the works of the usuliyyeen, the works of the literary figures, the works of the historians and poets etc. All jointly give us a picture what was going on in those periods.