If no CDN is used, it means the bootstrap.css file is served from your server.
So you can look in your theme's folder possible locations.
If CSS compression is not enabled on Configuration > development > performance > Aggregate CSS files, you can just right click on a page and view the page source to search for bootstrap.css, open the file (usually clicking on it). The first few lines will give you version number. You can probably inspect the bootstrap.js file included to get the used version.
If compression is enabled, you might be able to find the version from the corresponding bootstrap.js file. Right click and view the page source. At the bottom of the file, you will see the .js file that includes ALL the needed .js files.
Open it (clicking on it) and search for bootstrap.