Top bar -> 'Viewport Overlays' popup panel (object edimode popup panel).
Draw Xray addon will help you to draw retopo mesh on top of your highpoly mesh, without problems of build-in blender xray (no proper back-face culling).
How to use:
- grab addon from here Draw Xray
- install it by searching addon by name 'Xray'
- go to Properties panel -> shading - Draw Xray (blender 2.79)
After installation you will be able to see your mesh now and few more control sliders:
- Object offset - will determine how much your lowpoly object will be rendered in front of other objects. Set it to 0 to get default blender behavior.
- Polygon color opacity
- Edge /Vertex color opacity.
Update 2.3 Blender 2.8 only (23.08.2019)
- fixed crashes on scripts reload, and other bugs.
- by default per object snapping is now set to 'Global' . Saves few clicks: when you enable global snapping, all objects will use now global target. You can still change it per object in: 'Object Viewport' panel to: Disabled or Local (different snap target for each object)
Update 2.7 Blender 2.8 and 2.9 (19.11.2020)
- addon draws more uniform edges now
- more snapping options (based on shrink wrap modifier)
- Simplified UI (there is no longer separate panel in object properties)
Update 2.8 for Blender 2.93 and 3.0 (01.12.2021)
- Added compatibility with Blender 3.0
Update 2.9 for Blender 2.93 and 3.x (12.09.2022)
- Added compatibility with Blender 3.3
- remade snapping - only selected vertices will snap (in past - whole mesh would be snapped to high-poly obj)