Place something capacitive, like an apple.
Don't place anything that can scratch your screen.
If you have changed your 3D Touch sensitivity, set it below.
These iOS devices support 3D Touch:
iPhone 6s, iPhone 6s Plus, iPhone 7, iPhone 7 Plus, iPhone 8, iPhone 8 Plus, iPhone X, iPhone XS, iPhone XS Max.
Multi-Touch Weighing
(usually less accurate)