Test for the property you are searching, not the class.

package OP;
sub has_my_property { return 1; }

package BINOP;
sub has_my_property { return 0; }

package UNOP;
sub has_my_property { return 0; }
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etc. The Test::MockObject object can claim to have the property if it so desires.

I can't help you further without knowing why you need to know if it's exactly an OP or not.