Before when my knee would lock up and i would say my trainer would play with it. he was doing the Lachman test.

Lachman Test (most sensitive ACL test) 

• 98% accurate in predicting an ACL injury. 
  
  
  
  
  more info 
  http://orthosurg.ucsf.edu/patient-care/divisions/sports-medicine/conditions/knee/anterior-cruciate-ligament-injury-acl/