I used to work in an office that designed silos, which are prone to this problem. My boss said that dust thick enough to explode is very visible and thick appearing in the air. You would be choking on the stuff.

Rhed is right though, perhaps the fire department would be helpfull. Also, there are building code regulations on this for structures like silos and mills; maybe the nearest city hall would have information. If WHMIS (I assume a workplace safety beurocrats?) are involved, they should have written standards and a way to test for them.

If there is a chance you are in danger from this, act now. Dust produces awesome explosions. I've seen lots of pictures.

Good luck with this.