If it is there, and you can see it... Try some bracketing and decide the best exposure. But your eyes are more sensitive, so you need some luck and get a really thicker fog.
Depending on the angles, this could vary from one position to another, so move a bit.
Theese rays are diferent if they are made of dust or smoke, becouse a dust one depends on fewer particles. For indoors you could try adding aditional smoke with an artificial one. A smoke machine? Some incense perhaphs?
This also depends on the relationship of the light beam vs the surrounding darkness, and if you have a clear beam specially with irregularities. On a window, play with the angle of the incident light, make it the only source of light. This source light works if it is direct light.
If you have a really dark background you can increase the brightness of the beam in post pro.
Can I cheat? If everything else fails you can experiment with this: http://amicoperry.altervista.org/luce/index.html