I'm not aware of any way of checking server logs to see what SSL/TLS protocol was used for the connection for Windows server (its easy enough with Nginx and Apache).
Note using Google Analytics or the like is much better than trying to figure out the cryptic USER_AGENT field, though in theory that's another way of doing this and that will probably be logged in your server logs. See here for more details on how to go about doing it that way if you want: https://stackoverflow.com/questions/17798944/get-browser-name-and-version-from-iis-log-file-in-log-parser.
Once you have the browser and OS version of your visitors, you can look up this table to see if they support TLS 1.2:
https://en.m.wikipedia.org/wiki/Transport_Layer_Security#Web_browsers, and that should allow you to work out a rough percentage.
You can also use the ssllabs scanning tool (https://www.ssllabs.com/ssltest/) which will scan you site to test your SSL/TLS setup - including a telling you which version of TLS and which cipher a list of reference browsers will use. Highly recommend doing this scan anyway to see the state of your SSL/TLS config.
You will be mostly concerned with older versions of IE and older versions of android.
You could also do some browser detection on your website to add a warning to these users for a month or so before you switch off TLS 1.0 and 1.1. It's very easy to have a "[if lt IE 11]" statement to include a CSS style sheet which shows a warning for older IE version However IE10 no longer supports this syntax in standards mode and it is an affected browser. Also doing this for older Android browsers is not as easy.