Checking which version of Craft CMS a site is using
Looking via the FTP it appears that both Craft 3 and Craft 2 versions are installed in the vendors directory.


Craft 2 didn't use Composer, so wouldn't have a root vendor directory, so you're likely running some version of Craft 3.

Like Oli mentioned, if you can connect to the database, you can check the version column of the info table to see what version of Craft the site using.

If you can't access Craft's control panel, but can access the database, you can use this to gain control panel access: https://craftcms.stackexchange.com/a/13966/57