# 2 & 3 are the reverse sliding gears which have straight cut teeth. Every time your transmission jumps out of reverse it does a little more damage to the gear teeth. It's not long until the gear teeth look like # 3  order

# 4 Is the "Baulk ring" which must be replaced when the outer gear teeth worn like this one. Photo on the right is a close up of the inner Locking Angles. 

# 1 in the photo on the right show the locking angles. These should show no signs of wear and not appear bright and shiny like these. The gears are held in place by these locking angles and not by the selector forks. 

The two weakest parts in a type-1 Bug transmission or 1st gear and reverse, and they normally are the first to cause problems. It's not fool-proof but you can check 1st & reverse relatively easy. 
  Start the car and put it in low gear and let the clutch out all the way, then accelerate a little and back off the gas quickly so pressure is applied to both sides of the gear. Do this several times in low and in reverse gear with you hand off the shifter, if it doesn't jump out of gear then the chances are it's ok. 

Reverse Gear is for backing up and that's all. It's not strong enough for pulling some one out of the mud. If it starts jumping out of gear it does more damage each time until no reverse is available. If you hold the gear lever in reverse each time you back up it will most likely last for many months.
 When Reverse gear finely goes completely out don't be conned into just replacing the small reverse gear #2 or #3 in the above photo as this will only be a temporary fix. The Bulk ring locking angles are needed to hold reverse gear in mesh.

Low or 1st gear is also on the small size, but will handle a considerable amount of HP if you pre-load the gear train. In other words let out the clutch slightly until there is pressure on the gears and then put the hammer down as you let out the clutch. Rolling backwards and side stepping the clutch is sure disaster.