I have found some custom way, how you can detect your API version of ant-salesforce.jar. jar is an archive file, so it is possible to restore this file into components. Use WinRar or another archiver-program to do it.
After restoring you will thewill see the following structure of folders:
open next file META-INF\maven\com.force.api\force-wsc\pow.properties with any text editor. and here you will see version
#Generated by Maven
#Wed Dec 14 12:53:11 PST 2016
version=39.0.0
groupId=com.force.api
artifactId=force-wsc
For example, I have 39.0 API version of ant-salesfore. This is Spring '17