You have been told already that if you go back you will be charged with trespass. The possibility of them identifying you may be low, however, you do run a risk. Bear in mind if you make any payment via credit card, or any thing with your identification, their system may alert them.
Why tempt fate? You should stay away from the store. However,, the fact that you are walking around with a false SSN and asking about how hard it might be to detect you tell me that you have some larger issues you may need to address.
As stated before do not return to Macy's if you were told not to do so. If you come in contact with law enforcement and they have a reason to look into you background, it could land you in trouble. For example, if someone else chooses to shoplift that day and you are a witness or in the vicinity of trouble and police are called, they may look at your record as well. It is not worth risking a violation.
This answer does not constitute legal advice and does not create an attorney-client relationship.