The style of the reticle can vary. Common designs show the reticle as a simple dot or a crosshair.
The Google Cardboard guidelines offer this advice:
If you are concerned that displaying a reticle may reduce immersion or create unnecessary visual clutter, follow these guidelines1:
– Display the reticle only when the user approaches a target that they can activate.
– Project a light source, or design obvious hover states, for objects that the user can target.
1. ‘Interative Patterns: Display Reticle’ Google Cardboard Guidelines Website.