There are several ways to check which Sitefinity version is installed.
Go to Administration -> Version & Licensing (requires backend access):
Check the Sitefinity site's version from any frontend page generated by Sitefinity by inspecting the Generator meta tag
1. Open a Sitefinity generated page in the browser.
2. View the Page Source.
3. Check the content attribute of the <meta> tag named Generator:
<meta name="Generator" content="Sitefinity 8.1.5800.0 PU" />
To check the Sitefinity version from within Visual Studio:
The way to do this is to expand References in Visual Studio solution explorer and locate Telerik.Sitefinity assembly reference and select Properties. In Properties the Version matches the version of Sitefinity.