By reading the rules, combined with some common sense.
3. Do not advertise for your own shop within someone else's shop. This will lead to closure of your shop.
Making the statement that "by the way, this isn't advertising" is akin to killing someone and saying "by the way, that wasn't murder".
I would highly recommend shutting up and being glad that I'm not following through on the second sentence of that rule. Both of you.