I have been reading online that many famous duas recited by shias are in fact inauthentic even according to some of our scholars. what is the authenticity of duas like Kumayl, Tawassul, and Nudba to name a few and how can we be sure it has come from the Ahlul Bayt

Who said many of the duas we read are not authentic? Most of it authentic and reposted from either the holy prophet or Ahlul bait (as).
The above duas you mentioned are from Ahlul bait. The way to be sure of the authenticity of the duas is through our books of traditions such as Bihar al-Anwar, wasaail etc