Full Idea
If one can only know a priori that a proposition is necessary, then one can know only a priori that a proposition is contingent. The evidence relevant to determining the latter is the same as that relevant to determining the former.
Gist of Idea
If the necessary is a priori, so is the contingent, because the same evidence is involved
Source
Albert Casullo (A Priori Knowledge [2002], 3.2)
Book Reference
'Oxford Handbook of Epistemology', ed/tr. Moser, Paul K. [OUP 2002], p.113
A Reaction
This seems a telling point, but I suppose it is obvious. If you see that the cat is on the mat, nothing in the situation tells you whether this is contingent or necessary. We assume it is contingent, but that may be an a priori assumption.