Your question could have (atleast) three implications... (My guess
would be : 3)
1. You want to determine the version of the .NET FW programmatically
within your application.
2. You want to check the version before installing your program.
3. You just want to manually determine the version of the FW installed
on your own system.
Answers :
1. Query the System.Environment.Version property
2. A setup project built with the built-in template will automatically
check for this.