How do I extract the Unity version used to build

Given a certain build (for any platform), how can I tell which Unity version was used to build it?

When I want to fix a bug in an old game, I would like to fix it in the Unity version it was developed, to prevent additional bugs.

Look at the date of the player binary. As its never recompiled or changed by Unity you can look it up in the various unity installs you have and know what version it is