Cite the words of the Constitution which give Congress the DUTY to determine eligibility of Presidential candidates. It expressly says they have the duty to determine eligibility of THEIR OWN MEMBERS (whose qualifications are not dictated by the Constitution so it makes sense for them to handle that internally) but I don’t see anywhere that the same explicit authorization and command to determine Presidential eligibility is ever given to Congress. Show me the words.
FWIW, the qualifications for Congress are in the constitution, age, citizenship, and residency. There have been situations where elected members were challenged in the respective houses. Just saying, your "qualifications are not dictated by the Constitution" is flat out false.