LHS(determinant) of any Non-trivial FD in BCNF must be a Super key. It could also be a Candidate key as candidate key is also a super key. And Furthermore, if you observe, All such Non-trivial FDs where determinant is a Super key(But Not a candidate key) are having extraneous attributes on the LHS(i.e. in Determinant part) as the candidate key part of this super key itself can determine the extra attributes present in the determinant(LHS). So, It wouldn't be wrong to say that LHS(determinant) of any Non-trivial FD in BCNF must be a Candidate key But then you will have to first find the minimal FD set out of given FD set to apply this test for BCNF.
