Stop in for a cup of coffee

How do you test a tester? My sister has one of these ones. nc_oc=AQmzm36dX01jSRqDmBj4rpyVGDjxZMQLe-JD0Rac7IgZ9W25jgKYBom7EE3eMDNI4Ls&_nc_ht=scontent-iad3-1.jpg