Like a lot of other things (eg supination resistance, windlass function, and “overpronation”), the Lunge Test is something I talk about and write about a lot. The main reason being is that it is a clinical text that is supported by a lot of evidence regarding its reliability and validity, especially it link to injury. Not a lot of the clinical tests that are traditionally done tick as many boxes.
The lunge test is well documented online, but is a clinical weight-beairng test of the range of motion of the ankle joint. A couple of studies have now linked a limitation in it to injuries in footballers.
My preference is to actually do the test in shoes on top of any foot orthotics that they have to see if they can reach the required 35 or so degrees that is considered normal.