For TGMPA 2.5 and up, you’ll see the version used displayed on the bottom right-hand side of the TGMPA page.
For earlier versions, you’ll have to check the actual TGMPA file and look for the version number in the file header. Please do keep the above mentioned loading order in mind and search your WP install to see if there are several versions of TGMPA in use and check all version numbers.