A few people asked me after I wrote my article discussing among other things booster bags (often called magic bags or faraday bags) often used in shoplifting with particular emphasis on bypassing the security tags.
The quickest and I think the best way to test them is to do what shoplifters do, grab a few of your tags, and see whether your security poles will pick them up. Try looking for weak spots that the labels close to the poles, put the labels very low or straight, etc. What an experienced shoplifter often does is take a few labels from inside your shop. As no-one is looking for those labels, they are not hard to take, and as no-one is going to do anything to him/her coming into the shop so they can test your security all they like. Be worried if someone comes into your store and the security alarm goes off, its quite possibly not a malfunction but a test run.
As far as testing for booster bags, make one yourself and try it out with a few items. One of my clients found a person with a booster bag with over $240 worth of books of their books in it. A useful app "MD Faraday Bag Tester" that can help you test the bag and will give you a pretty good idea of how effective is available in the google play store
This app can also be used to check whether you have label security on an item. As a test, I went into a large shop, walked around with this app, tested many items and no-one said anything.
Update 05/07/2020: Details on how to make a booster bag to test your security system can be found here.
One of my clients was shown by a salesman of a security system that was claimed could detect if someone came into the shop with a booster bag and then went on to show that when a booster bag came into the shop that it set off the alarm. The idea is that the would-be thief hears the alarm and runs away.
However, when we tested it with the booster bag as above, it did not set off the alarm. I am sure it has to do with the density of the material. Our booster bag may not have been as dense as his. The problem here is that many items use aluminum, you cannot have an alarm going off every time a pram comes into the shop.
So I would suggest anyone thinking of purchasing such an alarm system to test it with their own booster bag.