Let's assume you have two layers, Layer 1 (the image you want to match), and Layer 2 (the image you want to liquify). Obviously before you start make sure both faces are the same size and in the same general position.
Select Layer 2 in the layers panel, and click
When the Liquify window opens, scroll down the options on the right, and choose
Show Background. Set the
Use option to
Layer1. You can change the opacity by moving the slider, or you can toggle backwards and forwards by selecting/deselecting the
Show Background option.
This example is for Photoshop CC 2017