Here is declaration for cvDet in header:
double cvDet(const CvArr* mat);
It should give a clue.
Here is documentation for cvDet - another clue.
Returns the determinant of a square floating-point matrix.
Try this:
double A[]={1,2,3,4};
CvMat *srcA=cvCreateMat(2, 2, CV_64FC1);
srcA->data.db = A;
cout << cvDet(srcA) << endl;