All lash adhesives contain one interesting substance – cyanoacrylate. This thing is responsible of the glue's ability, well, to glue and is an irritant. If a manufacturer of your adhesive says that their glue is hypoallergenic, then you should pause and think. There are NO hypoallergenic adhesives and such declarations are just another one marketing trick.
If you do a patch test on skin, then it will most likely cause an allergy or irritation. Yes, this may be very easy and unnoticeable for your client but still her immune system will remember it and later your client will have a stronger reaction when the irritant touches her skin. If such a test did cause an allergy, then this client is now lost to the extension because allergies do not get better over time. I'll say that again – EVERY adhesive contains this irritant.
Usually manufacturers recommend doing the following test 48 hours before the extension: extend a pair of the outer corner lashes using those materials you're going to use during the procedure. After the first contact with such a small amount of products, you can avoid dangerous outcome.
But you need to remember that no patch test can 100% save your clients from possible allergies because they can show up many years after using some materials. By the way, such a test can help you understand how the lashes are growing and which effects fit your client better.