Nothing except for that I trust that the President of the Senate, Dick Cheney (R) and the Senate Majority Leader Bill Frist (R) would have checked the credentials and eligbilty of Freshman Senators in January of 2005 when Obama entered the Senate.
However, if you have evidence that Cheney and Frist did not do their due diligence of requiring that freshman Senators show their credentials and eligibility, I would like to see that evidence.
Now you are the one asking for evidence/proof of a negative.
I would think that there should be evidence that such is routinely done. If it is, then it was probably done in Obama's case.
But since there is apparently no one responsible for checking the qualification of someone to be President, I would not expect that anyone does it for Senators. Clearly the Senate has the power to do that, it's in the Constitution's Article I, Section. 5.
Each House shall be the Judge of the Elections, Returns and Qualifications of its own Members,
But they apparently only do it on an "exception" basis. Thus I see no problem with making an exception of Barack Hussien Obama, who acknowledges having a foreign citizen father and step father, and who has been very "close" with his records, such as college records as well as others, as compared with other recent Presidents-elect.