What should be done before using a fireplace that has not been used in decades?
You hire a chimney sweep to inspect (and clean) it. In some cases it may need to be completely rebuilt - in others, it may simply need cleaned and inspected, or it may need something in the middle.

At that age, you may well find that you'll face relining to make use of it under modern conceptions of what's safe - it will have deteriorated with age and probably was not built to standards that are considered safe these days in the first place. 

There are ways to solve that problem, but which way is applicable will vary with the state of the chimney, and any work that might have been done to it in the 70 years before it was disused for 20 years, as well as what's been going on inside it for 20 years.