Which action might best help identify a worn U-joint?

Identifying a worn U-joint is often accomplished by checking for lateral play. A worn U-joint will exhibit excessive movement when manipulated, which can manifest as play in the joint itself. When you grasp the driveshaft near the U-joint and attempt to move it side-to-side or up-and-down, any noticeable movement indicates wear. This lateral play can lead to drivability issues, such as vibrations or noises during driving, making it an effective method for determining the condition of the U-joint.

In contrast, while squeezing the U-joint may provide some feedback about its stiffness, it is not a definitive test for wear. Listening for grinding sounds can indicate a problem, but might not specifically point to a U-joint issue since noises could originate from other components in the drivetrain. Examining for surface rust can help gauge general condition or exposure to moisture, but it doesn't effectively reveal wear or mechanical play within the joint itself. Therefore, checking for lateral play is the most reliable method for identifying a worn U-joint.