Where in the CSM yaw is the middle gimbal, roll is the middle gimbal in the LEM. You should see a red shaded area on either side of the FDAI's edges (on the panel itself, not the 8ball like in the CSM). If the double line running vertically down the FDAI touches this red band on both ends you have gimbal lock.
Visually, you would have gimbal lock if the Lem were at 0,0,0 RPY and you rolled the LEM onto one side.
Hope this helps! I could link some pictures if it's still unclear.