The grasslands of the Midwestern united states have been grazed and farmed so that almost no grasslands remain with their original species of plants and animals. How could people in the Midwest determine what kind of natural ecosystem they are in? Check yearly average rainfall
Look around, since even a changed ecosystem is still an ecosystem.
Look in history books to determine what type of organisms lived there before.
Check yearly average temperatures and see what types of plants and animals live in those temperatures.