Depending on your client, there are two ways to perform a patch test.  Using your desired pigment selection, make a few hair strokes behind the ear, near the hairline.  Just as you would when microblading.  You can also implant a stroke in this manner within your client's brow (towards the front) if they are comfortable with this.

Within 24 hours you will know if your client is allergic or not.  If they are allergic, they can react in various different ways: red, itching, hot to the touch, etc.