Displaying semi-transparent objects in OpenGL 3D view?

This might be a basic question but when you’re say modeling the head lights of a car and you want to see both the headlight lens and the bulb and reflector array behind the lens, how do you make the lens object semi-transparent in solid view?

I could use cycles to preview but I want to be able to see all my objects as I edit them.

Thanks. I knew the functionality would be somewhere.