There are several ways to check which Sitefinity version you have installed:
First way:
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 3.2.1616.2:0" />

Second way:
1. Open the project ~/bin folder.
2. Find the Telerik.Cms.dll assembly.
3. Right click on the assembly file.
4. Go to Properties --> Details tab.
5. Check the File version property value.

You can also check your current build version directly using the Sitefinity database:
1. Open your Sitefinity database.
2. Find the sf_CmsVariables table.
3. Check the SchemaVersion value.

SchemaVersion value = xxxx

Below is some description of the releases:

 

Release Sitefinity 3.7 SP3
3.7.2096.2 - standard
3.7.2096.3 - community

    
for .NET 2.0

3.7.2096.220 - standard
3.7.2096.320 - community

Release Sitefinity 3.7 SP2
3.7.2057.2 - standard
3.7.2057.3 - community
    

for .NET 2.0   
3.7.2057.220 - standard
3.7.2057.320 - community

Release Sitefinity 3.7
3.7.1990.2 - standard
3.7.1990.3 - community

    

for .NET 2.0

Release Sitefinity 3.6 SP2
3.6.1936.2 - standard
3.6.1936.3 - community

for .NET 2.0   
3.6.1936.220 - standard
3.6.1936.320 - community

Release Sitefinity 3.6 SP1 Hotfix
3.6.1927.2 - standard
3.6.1927.3 - community

for .NET 2.0    
3.6.1927.220 - standard
3.6.1927.320 - community

Release Sitefinity 3.6 SP1
3.6.1920.2 - standard
3.6.1920.3 - community

for .NET 2.0    
3.6.1920.220 - standard
3.6.1920.320 - community

Release Sitefinity 3.6 hotfix
3.6.1870.2 - standard
3.6.1870.3 - community

for .NET 2.0    
3.6.1870.220 - standard
3.6.1870.320 - community
Release Sitefinity 3.6
3.6.1861.2 - standard
3.6.1861.3 - community

for .NET 2.0    
3.6.1861.220 - standard
3.6.1861.320 - community
Release Sitefinity 3.5 SP1
3.5.1747.2 - standard
3.5.1747.3 - community
Release Sitefinity 3.5
3.5.1714.2 - standard
3.5.1714.3 - community
Sitefinity 3.2 SP2 Hotfix
3.2.1616.2 - standard
3.2.1616.3 - community
 Sitefinity 3.2 SP2
3.2.1598.2 - standard
3.2.1598.3 - community
 Sitefinity 3.2 SP1
3.2.1526.2 - standard
3.2.1526.3 - community
 Sitefinity 3.2
3.2.1500.2 - standard
3.2.1500.3 - community
 Sitefinity 3.1 SP2
3.1.1473.2 - standard
3.1.1473.3 - community