How can anyone tell anything from ashes?
Probably the incompleteness of the process. Cremation requires temps north of 1400F and a good period of time to render everything (teeth, bones) into ash. Early on the cells have to burst as the water (which is most of our body mass) rises in temperature. After the dessication other stuff evaporates or maybe ignites.
Crematoria are the modern way of doing this. A Viking-style ship burning might get most of the job done.
This is why I suspect the Battle of Watling Street (where Boudica led 70,000 of her fellow genocidal idiots to their sorry ends) probably wound up resulting in a big bonfire just to get rid of the dead, which took care of most of the mass (the water) but probably left behind over 2 million teeth. That could be the way to identify the battle site, and with luck, allow for DNA testing of the remains.