This has been discussed on this forum before. It depends on what sort of building has been burning, what it contained, was there any asbestos, how the fire was put out and other things. So you will need a risk assessment which will include substances hazardous to health but might also include asbestos. The heat of the fire would have driven away/consumed the volatile organic compounds but the amount of dust would depend on if the area had dried out and how it was cleared. Machinery might raise more dust than doing it by hand.  

Essentially there are loads of factors to consider and for some unless you test to the nth degree it might be simply be better to assume the worst and work on that.