Hello,
The fastest way is checking your web.config file. If you take a look at following <system.web> section you will see the control reference for System.Web.UI. For the .NET 2.0 is as follows:
<add tagPrefix="asp" namespace="System.Web.UI" assembly="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" />
Whereas the .NET 3.5 line differs with the Version parameter like this:
<add tagPrefix="asp" namespace="System.Web.UI" assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" />
Best regards
Ondrej Vasil