When PP sub is called the CX for this sub is created.
By accessing this context by CxHASARGS(CX_CUR()) we can check this sub hasargs or not.
But for the XSUB context is not created, so we can not use this trick.
Is is possible to know XSUB was called using &sub or not?
UPD
Looking at sources at pp_entersub when cv is XSUB. We can see how code handle hasargs. It access PL_op->op_flags & OPf_STACKED then a bit later call the target XSUB:
if( PL_op->op_flags & OPf_STACKED ...
...
CvXSUB(cv)(aTHX_ cv);
Will it be OK if I in my XSUB I do same check of PL_op->op_flags & OPf_STACKED?