You could create a reference, then destroy it.

SV * rv = SvRV_noinc(sv);
int blessed = sv_isobject(rv);
SvREFCNT_dec(rv);
[download]

or you could use the equivalent

int blessed = SvOBJECT(sv);
[download]

I don't know if that's kosher, but I don't see it breaking.