How can I find which version of the Data Exchange Framework I have installed?
You can check your log files - Sitecore logs the information about installed Sitecore Modules when the application starts:
One more option: assuming that you installed DEF in that environment you will see that information in
core database under
/sitecore/system/Packages/Installation history/Data Exchange Framework node. There will be one or more nodes with all the information about the versions of DEF which were installed there:
It will not work if you installed DEF in one environment and then moved it to another via deploy with item synchronization (e.g. Unicorn or TDS) and files published to the new environment.